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LIBRARY 

OF    THE 

UNIVERSITY  OF  CALIFORNIA. 


LIBRARY 


VARIATION 


IN    ANIMALS    AND    PLANTS 


BY 


H.  M.  VERNON,  M.  A.,  M.  D. 

Fellow  of  Magdalen  College,  Oxford 


NEW  YORK 

HENRY  HOLT  AND  COMPANY 
1903 


GENERAL 


COPYRIGHT,  1902, 

BY 
HENRY   HOLT  &  CO 


T 


BIO1 
RA 
6 


PKEFACE. 

IN  this  little  book  I  have  endeavoured  to  give  a  brief 
account  of  the  subject  of  "  Variation,"  so  far  as  the 
present  state  of  our  knowledge  admits.  Though  I  did 
not  in  any  degree  aim  at  giving  a  complete  representa- 
tion of  the  subject,  yet  I  hope  that  most  of  the  more 
important  and  more  recent  work  has  been  included.  I 
have  not  treated  Variation  of  Plants  so  fully  as  that  of 
Animals,  and  from  lack  of  thorough  acquaintance  with 
the  literature,  have  probably  made  some  omissions  of 
real  importance  to  the  adequate  comprehension  of  the 
subject  in  its  bearing  on  living  organisms  taken  as  a 
whole.  For  such  I  offer  my  apologies.  I  have  pur- 
posely avoided  any  higher  mathematics  in  discussing 
the  facts  of  variation,  as  it  seemed  out  of  place  in  a  book 
of  this  character. 

An  obvious  criticism  upon  the  contents  of  the  book 
will  be  that  I  have  given  greater  prominence  to  my  own 
researches  than  their  intrinsic  importance  warrants. 
To  this  I  frankly  plead  guilty,  urging  in  extenuation 
that  I  did  not  intend  to  write  a  text-book  in  the  ordi- 
nary sense  of  the  term.  Thus  some  of  the  hypotheses 
and  interpretations  of  facts  which  I  have  given  are  my 
own  personal  opinions,  and  by  no  means  current  views 
held  in  general  acceptation.  Also  a  few  of  the  data 
published  in  the  latter  part  of  Chapter  VI.,  and  those 
on  "  identical  twins  "  in  Chapter  IV.,  are  here  published 

m 


11270? 


iv  PREFACE. 

for  the  first  time,  and  are  for  this  reason  given  rather 
in  extenso. 

As  regards  the  illustrations,  I  am  indebted  to  the 
Royal  Society  for  the  blocks  of  Figs.  17,  18,  20,  and  21, 
whilst  Figs.  3,  6,  9,  16,  19,  and  25  have  been  copied 
from  illustrations  in  their  publications.  Figs.  22,  23, 
and  24  are  copied  from  Davenport's  "  Experimental 
Morphology,"  and  Figs.  7  and  8  from  Bateson's  "  Ma- 
terials for  the  Study  of  Variation."  The  rest  are 
either  original,  or  from  sources  indicated  in  the  text. 

I  desire  to  take  this  opportunity  of  expressing  my  ob- 
ligations to  Mr.  E.  S.  Goodrich  for  his  kindness  in  read- 
ing over  the  manuscript,  and  to  Professors  W.  F.  R. 
Weldon  and  S.  H.  Vines,  for  their  useful  advice  and 
suggestions. 


CONTENTS. 

PART  I. 
THE  FACTS  OF  VARIATION. 

CHAPTER  I. 
THE  MEASUREMENT  OF  VARIATION. 

PAGE 

Variation    studied   from  the  mathematical   standpoint— Vari- 
ation of  birds  diagrammatically  represented — Distribution 
of  crab  measurements — Normal  curve  of  error — Its  relation  ^ 
to  binomial    curve — Variations   in   animals    and    plants 
subject  to  Law  of  Frequency  of  Error — Measurement  of 
variation  in  terms  of  Probable  Error,  Arithmetic   Meant 
Error,  and  Error   of  Mean  Square— Examples  of  Asym-  / 
matrical  series — Representation  of  these  by  a  generalised 
mathematical  expression, 1 

CHAPTER  H. 
DIMORPHISM  AND  DISCONTINUOUS  VARIATION. 

Dimorphism  in  the  earwig  and  in  the  crab — How  to  dis- 
tinguish between  species  and  varieties,  as  instanced  by 
dimorphism  in  certain  fishes,  and  in  a  marsh  plant— Poly- 
morphism in  plants — Series  of  Fibonacci — Discontinuous 
variation  in  animals  as  regards  vertebrae,  ribs,  mammae, 
teeth,  digits,  and  other  characters — Homceosis — De  Vries' 
Theory  of  Mutation — Dimorphism  may  be  due  to  internal 
causes,  or  the  result  of  divergent  evolution— Physiological 
Selection— Infertility  between  varieties,  .  ,  .37 


vi  CONTENTS. 

CHAPTER  III. 
CORRELATED  VARIATIONS. 


The  measurement  of  correlation — Galton's  function — Cor- 
relation between  various  organs  in  man,  in  local  races  of 
the  shrimp,  and  in  crabs — Comparison  between  primitive 
and  civilised  races  of  man — Correlation  between  morpho- 
logical characters  and  the  reproductive  system — Genetic 
Selection  in  man — Especial  fertility  of  type  forms  in  cer- 
tain plants — Evolution  in  the  Peppered  moth — Parallel 
variation — Importance  of  mathematical  treatment  of  varia- 
tion,   72 

PART    II. 
THE  CAUSES  OF  VARIATION. 

CHAPTER  IV. 
BLASTOGENIC  VARIATIONS. 

The  ultimate  cause  of  blastogenic  variation— Effect  of  stale- 
ness  and  of  comparative  maturity  of  sex-cells  on  the 
characters  of  organisms— Amphimixis — Identical  twins — 
Transplantation  of  ova  in  the  rabbit — Law  of  Ancestral 
Heredity  in  man  and  in  the  Basset  hound— Regression 
towards  mediocrity — Exclusive  inheritance — Homotyposis,  101 

CHAPTER  V. 
BLASTOGENIC  VARIATIONS  (Continued). 

Reversion;  commonest  in  crossed  races,  as  of  the  pigeon  and 
fowl;  its  theoretical  explanation — Prepotency;  in  the 
trotting  horse  and  in  man;  probably  due  in  large  part  to 
inbreeding—Mendel's  Law  of  Hybridisation,  and  its 
range— Natural  and  artificial  plant  hybrids— Animal 
hybrids — Sports  ;  probably  of  different  origin  to  normal 
variations— Artificial  production  of  monsters— Telegony; 
probably  non-existent — Parthenogenesis  in  an  Ostracod 
and  in  Daphnia— Does  sexual  reproduction  induce  vari- 
ability?—Relation  of  variability  of  individual  to  variability 
of  race— Asexual  reproduction  in  plants— Bud-variation,  138 


CONTENTS.  vii 

CHAPTER  VI. 
CERTAIN  LAWS  OF  VARIATION. 

PAGE 

Effect  of  environment  on  growth  diminishes  rapidly  from 
time  of  impregnation  onwards — Reaction  of  an  organism  to 
environment  dependent  on  nature  of  organism— Rapidly 
diminishing  rate  of  growth  in  man  and  in  the  guinea-pig 
with  progress  of  development — Variability  also  diminishes 
with  growth — Effect  on  growth  once  produced,  probably 
never  eradicated — Increased  variability  of  sparrow  and  of 
periwinkle  in  America — Relation  between  variability  and 
want  of  adaptation  to  environment — Variability  of  migra 
tory  and  non-migratory  birds— Does  domestication  increase  1  Y\ 
variability? 3fSKF 

CHAPTER  VH. 
THE  EFFECT  OF  TEMPERATURE  AND  OF  LIGHT. 

Variations  and  modifications — Effect  of  temperature  on  growth 
of  frog — Optimum  temperature  of  growth  in  plants — 
Effect  of  temperature  on  size  of  sea-urchin  larvae,  of 
Lepidoptera,  and  of  Mollusca — Seasonal  dimorphism  in 
certain  Lepidoptera  in  its  relation  to  temperature — Temper- 
ature differences  giving  rise  also  to  local  races,  sports, 
and  phylogenetic  forms — Critical  period  of  reaction  to 
temperature — Effect  of  Arctic  climate  on  coat  of  mammals — 
Effect  of  darkness  and  of  light  on  growth  of  plants— Effect 
of  sunlight  and  of  diffused  light— How  far  does  pigmen- 
tation of  animals  depend  on  exposure  to  light? — Cave 
animals — Illumination  of  under  surface  of  flounder — Effect 
of  light  and  of  darkness  on  Molluscs— Variable  protective 
resemblance  in  the  frog,  in  fish,  and  in  larvae  and  pupae  of 
certain  Lepidoptera, 223 

CHAPTER  VIII. 
THE  EFFECT  OF  MOISTURE  AND  OF  SALINITY. 

Effect  of  humidity  of  soil  on  plant  growth — Effect  of  dry 
and  moist  surroundings  on  characters  of  plants — Desert 
plants  and  Aquatic  plants— Effect  of  moisture  on  Lepi- 


viii  CONTENTS. 

PAGE 

doptera  and  on  Molluscs — Characters  of  maritime  plants 
probably  due  to  saline  environment — Conversion  of  A. 
salina  into  A.  milJiausenii  and  into  Branchipus — Effect  of 
increased  salinity  on  characters  of  the  cockle — Influence  of 
salinity  on  rate  of  growth  of  Tubularians,  and  on  size  of 
sea-urchin  larvse, 260 

CHAPTER  IX. 

THE  EFFECT  OF  FOOD  AND  OF  PRODUCTS  OF 
METABOLISM. 

Effect  of  artificial  manures  on  growth  of  crops — Effect  of 
nutrition  on  plant  variation — Development  of  bees  and 
of  aphides  in  relation  to  food — Influence  of  nature  of  food 
on  wing  markings  of  certain  Lepidoptera — Dependence  of 
colour  of  larvae  on  plant  pigments — Influence  of  food  on 
growth  of  tadpoles — Plumage  of  certain  birds  altered 
by  abnormal  diet — Quality  of  food  influences  organs  of 
digestion — Every  organism  probably  has  specific  metab- 
olism, which  has  especially  adverse  action  on  its  own 
growth — Products  of  metabolism  may  stimulate  growth- 
Effects  of  small  quantities  of  urea,  uric  acid,  and  ammonium 
salts — Influence  of  volume  and  of  surface  area  of  water  on 
growth  of  pond  snail — Influence  of  surface  area  on  growth 
of  tadpole — Effects  of  increasing  quantities  of  metabolic 
products  on  characters  of  a  snail,  and  of  a  Crustacean,  .  281 

CHAPTER  X. 
THE  EFFECTS  OF  CONDITIONS  OF  LIFE  IN  GENERAL. 

Local  conditions  of  life  perhaps  the  cause  of  local  races,  but 
proof  of  this  as  a  rule  impossible — American  and  European 
trees  compared — Alpine  and  Arctic  plants — Effects  of 
cultivation — Local  races  of  oysters  and  of  snails— Lepi- 
doptera in  Malay  Archipelago — Local  races  of  shrimps,  of 
mackerel,  and  of  herring — North  American  birds  and 
mammals — Action  of  climate  on  goats  and  on  rabbits — 
Effect  of  domestication  on  rabbits,  pigeons,  fowls,  and 
ducks, 310 


CONTENTS.  ix 

PART    III. 
VARIATION  IN  ITS  RELATION  TO  EVOLUTION. 

CHAPTER  XI. 

THE  ACTION  OF  NATURAL   SELECTION  ON 
VARIATIONS. 

PAGE 

Proof  of  Natural  Selection  in  the  crab,  and  in  the  sparrow — 
Selection  in  man — Evolution  of  the  mouse — Inheritance  of 
acquired  characters  seems  to  be  shown  by  cumulative  effects 
of  conditions  of  life,  as  European  climate  acting  on  Ameri- 
can maize;  domestication  acting  on  wild  turkeys  and  ducks; 
changed  climate  acting  on  sheep  and  dogs — Environment 
may  act  on  germ-plasm  through  specific  excretions  and 
secretions — Cases  of  inherited  effects  of  use  and  disuse,  and 
of  epilepsy,  accounted  for — Somatic  variations  may  increase 
variability,  and  so  afford  Natural  Selection  a  better  handle 
to  work  upon,  .  .  .  *' 335 

CHAPTER  XII. 
ADAPTIVE  VARIATIONS. 

Adaptability  a  fundamental  property  of  protoplasm— In- 
stances of  adaptive  variations  in  plants — Acclimatisation 
of  Protozoa  to  high  temperature,  to  poisons,  to  mechanical 
stimuli,  to  saline  solutions— Acclimatisation  of  fresh-water 
Mollusca  to  salt  water,  and  of  various  marine  animals  to 
fresh  water — Acclimatisation  of  Mammals  to  vegetable 
poisons,  and  to  toxins — Sum  total  of  somatic  variations 
always  in  direction  of  adaptation — Somatic  variations  of 
importance  in  evolution,  but  they  can  effect  little  without 
Natural  Selection— Germinal  Selection,  .  .371 


VARIATION  IN  ANIMALS  AND 
PLANTS. 


PART  I. 
THE  FACTS   OF  VAKIAT1OK 

CHAPTEK  I. 

THE  MEASUEEMENT  OF  VARIATION. 

Variation  studied  from  the  mathematical  standpoint — Variation  of 
birds  diagraramatically  represented — Distribution  of  crab  measure- 
ments— Normal  curve  of  error — Its  relation  to  binomial  curve — 
Variations  in  animals  and  plants  subject  to  Law  of  Frequency  of 
Error — Measurement  of  variation  in  terms  of  Probable  Error, 
Arithmetic  Mean  Error,  and  Error,  of  Mean  Square — Examples  of 
asymmetrical  series — Representation  of  these  by  a  generalised 
mathematical  expression. 

IF  a  number  of  individuals  of  any  species  be  com- 
pared, it  will  be  found  that  they  all  show  differences 
from  each  other  either  in  size,  shape,  colour,  relation 
of  parts,  or  other  characteristics;  in  fact,  no  two  of 
them  are  exactly  alike.  Even  if  offspring  be  compared 
with  their  own  parents,  similar,  though  on  the  whole 
not  such  marked,  differences  will  present  themselves. 
These  differences  constitute  what  is  known  as  Varia- 
tion, and  it  is  into  the  facts  of  this  variation,  and  its  im- 


2  THE  MEASUREMENT  OF  VARIATION. 

portance  as  the  corner  stone  of  the  whole  fabric  of  Evo- 
lution, that  we  shall  briefly  inquire  in  the  following 
pages. 

In  his  "  Origin  of  Species,"  Darwin  clearly  recog- 
nised the  fundamental  importance  of  the  existence  of 
variation,  for  without  it  there  could  evidently  be  no 
such  thing  as  evolution.  In  his  "  Variation  of  Animals 
and  Plants,"  also,  he  brought  together  an  enormous 
mass  of  material  concerning  the  facts  of  variation, 
though  unfortunately  this  dealt  almost  exclusively  with 
organisms  in  a  condition  of  domestication.  Still,  there 
was  sufficient  evidence  even  then  to  show  that  wild  ani- 
mals and  plants  are  also  subject  to  variation,  though 
Darwin  probably  did  not  fully  recognise  how  consider- 
able and  universal  this  variation  is.  As  to  the  causes 
of  variation,  Darwin  did  not  hazard  many  conjectures. 
To  do  so  would  have  been  premature^  and  from  actual 
lack  of  knowledge  almost  impossible.  For  many  years 
after  the  publication  of  Darwin's  work,  the  additions  to 
our  knowledge  of  the  subject  of  variation  were  exceed- 
ingly small.  Scientists  seemed  to  rest  content  with  the 
material  he  had  collected,  and  to  theorise  on  this  alone, 
rather  than  to  test  their  theories  by  a  search  after  fresh 
facts  and  data.  Within  the  last  decade,  however,  the 
importance  of  the  scientific  study  of  variation  has  be- 
gun to  be  more  thoroughly  recognised,  and  has  resulted 
in  its  being  attacked  with  considerable  vigour  from  sev- 
eral entirely  different  points  of  view.  Investigations 
from  the  mathematical  side  have  shown  that  many  of 
the  apparently  disconnected  facts  of  variation  can  be 
expressed  with  ease  and  lucidity  by  exact  mathematical 
expressions,  and  that  much  material  which  has  hitherto 


THE  MEASUREMENT  OF  VARIATION.  3 

been  regarded  as  quite  outside  all  law  was  in  reality 
amenable  to  treatment  according  to  the  well-known. 
Laws  of  Chance.  Again,  investigations  from  the  ex- 
perimental side  have  suggested  much  concerning  the 
causes  of  variations,  both  genetic  and  somatic.  Still 
again,  a  fresh  burst  of  activity  in  the  collection  of  data 
regarding  the  actual  facts  of  variation,  more  especially 
in  respect  of  organisms  found  in  a  state  of  nature,  has 
shown  us  how  much  in  this  branch  of  the  subject  there 
remains  for  us  yet  to  learn. 

Perhaps  the  keynote  of  most  of  the  recent  work  on 
variation  lies  in  the  recognition  of  the  fact  that  almost 
all  the  problems  to  be  solved  must  be  attacked  from  a 
numerical  standpoint.  It  is  no  longer  sufficient  to 
say  that  such  and  such  a  kind  of  variation  is  frequently 
or  occasionally  found.  It  is  necessary  to  know  the 
exact  amount  of  the  variation,  so  far  as  it  is  measur- 
able, and  the  exact  proportion  of  cases  in  which 
it  occurs.  Only  by  obtaining  data  of  this  kind 
can  we  hope  to  ascertain  with  any  certainty  the 
probable  degree  of  importance  of  any  particular  vari- 
ation in  the  evolution  of  a  species,  and  whether  such 
evolution  is  actually  taking  place  at  the  present 
day.  No  apology  is  therefore  needed  for  the  fre- 
quent introduction  of  figures  into  the  study  of  ques- 
tions of  variation.  Rather  is  this  necessary  if  one 
should  attempt  to  found  theories  and  deduce  conclusions 
from  generalised  statements  and  opinions,  unsupported 
by  such  evidence.  To  say  that  any  particular  organ  is 
very  variable  means  but  little,  for  so  much  depends 
upon  the  personal  opinion  of  the  observer  as  to  what 
constitutes  a  great  and  what  a  slight  variation.  But 


4  THE  MEASUREMENT  OF  VARIATION. 

supposing  it  be  said  that  out  of  a  large  number  of  indi- 
viduals half  varied  in  size  by  ±  5  per  cent,  from  the 
average  of  the  whole,  then  there  is  afforded  a  numerical 
expression  of  the  degree  of  variation,  which  can  readily 
be  compared  with  similar  expressions  concerning  the 
variability  of  other  parts  of  the  same  organism,  and 
with  those  of  quite  distinct  organisms. 

Let  us  first  of  all,  therefore,  examine  one  or  two 
simple  series  of  measurements  made  on  a  group  of  indi- 
viduals of  a  species,  so  as  to  get  some  idea  of  the  actual 
differences  exhibited  by  the  varying  characters,  or,  as 
they  have  been  termed,  the  variants.  Some  of  the  most 
striking  are  those  obtained  by  J.  A.  Allen,*  concerning 
the  variation  in  certain  mammals  and  winter  birds  of 
East  Florida.  Of  a  species  of  squirrel  (Sciurus  caroli- 
nensis),  for  instance,  28  individuals  were  measured, 
and  these  .  neasurements  are  reproduced  to  scale  in  the 
accompanying  diagram.  Here  the  animals  are  ar- 
ranged in  order  according  to  the  length  of  their  body 
in  inches,  and  the  corresponding  values  for  the  head, 
tail,  and  forefoot  are  given  on  the  same  ordinates.  By 
means  of  this  diagram,  the  magnitude  of  each  and  all 
of  the  measurements  made  can  be  read  off  at  a  glance. 
The  body  was  on  an  average  9.15  inches  long,  but  the 
extreme  values  were  8.25  and  10.20  inches,  or  respect- 
ively 9.8  per  cent,  and  11.5  per  cent,  less  and  greater 
than  the  mean.  The  tail  measurements  were  even 
more  variable  than  this,  the  extremes  varying  from  6.75 
to  8.75  inches,  or  by  respectively  14.3  per  cent,  and  11 
per  cent,  from  the  mean.  In  the  forefoot  the  range  of 
variation  was  less,  and  in  the  head  smaller  still;  but 
*  Bulletin  Museum  Comp.  Zo51,,  Harvard,  1871. 


Body 


Tail 


Head 


Fore  foot 


FIG.  1. — Variation  of  Sciurus  carolinensis. 


6  THE  MEASUREMENT  OF  VARIATION. 

there  was  never  any  constancy,  every  animal  varying 
in  respect  of  each  of  the  measurements  made.  This 
is  a  point  of  fundamental  importance,  which  cannot  be 
too  thoroughly  grasped.  Every  organism  varies  in  re- 
spect of  all  its  characters,  whatever  be  their  nature. 
The  amount  of  this  variation  differs  greatly,  as  these 
results  well  show,  but  it  is  always  present  in  a  greater 
or  less  degree.  Another  fact  which  this  diagram  brings 
out  very  clearly  is  the  comparative  independence  of 
these  measurements.  Because  the  body  of  one  animal 
is  longer  than  another,  it  by  no  means  necessarily  fol- 
lows that  the  head  or  tail  is  longer  also.  A  superficial 
glance  at  this  diagram  might,  indeed,  lead  one  to  sup- 
pose that  the  various  parts  of  the  body  were  absolutely 
independent  of  each  other.  But  this  we  know  not  to 
be  the  case.  Between  most  parts  and  organs  there  is  a 
greater  or  less  degree  of  correlation,  so  that,  on  an  aver- 
age, animals  with  a  longer  body  may  have  a  longer  head 
and  longer  tail  than  animals  with  a  shorter  body.  A 
careful  examination  of  the  diagram  will  show  that,  on 
the  whole,  though  with  numerous  exceptions,  the  curves 
for  head,  tail,  and  foot  do  slope  very  slightly  upwards 
from  left  to  right,  though  nothing  like  as  much  as  the 
curve  for  body  lengths.  Some  degree  of  correlation  is 
therefore  present,  though  it  is  only  slight.  We  know 
that  frequently  it  may  be  very  great  indeed,  as  for  in- 
stance between  the  two  fore  limbs  or  two  hind  limbs  of 
a  quadruped,  and  very  considerable  between  a  fore  and 
a  hind  limb;  but  into  this  question  we  must  not  enter 
now. 

Almost  innumerable  diagrams  of  a  similar  nature  to 
the  above  might  be  given,  but  this  is  scarcely  neces- 


THE  MEASUREMENT  OF  VARIATION.  7 

sary.  All  that  they  would  demonstrate  would  be  the 
fact  that  variation  of  a  similar  nature — though  of  a 
varying  degree — is  present  in  all  organisms,  to  what- 
ever class  of  the  Animal  or  Vegetable  Kingdom  they 
belong.  Should  more  evidence  of  this  kind  be  de- 
sired, the  reader  is  referred  to  Wallace's  book  on  "  Dar- 
winism "  (Chapter  III).  Here  an  admirable  series  of 
diagrams  is  given,  illustrating  the  variation  in  several 
species  of  lizards,  birds,  and  mammals.  The  diagram 
given  above  is  modelled  on  the  plan  adopted  by  Wal- 
lace, and  still  earlier  by  Galton,  as  the  one  best  adapted 
for  bringing  before  the  eye  the  facts  of  individual 
variability. 

In  the  above  diagram  the  measurements  of  only  28 
different  individuals  are  given,  and  hence  we  are  not 
able  to  gather  much  as  to  the  distribution  of  the  differ- 
ent measurements  about  their  means.  Supposing  that 
instead  of  tens,  fifties  or  hundreds  of  the  animals  had 
been  measured,  what  should  we  expect  to  find?  Would 
there  or  would  there  not  be  just  as  many  animals  with  a 
very  long  or  very  short  body  length,  as  with  a  moder- 
ately long  or  moderately  short  one,  or  as  with  a  nearly 
average  one?  Such  a  question  as  this  is  also  best  an- 
swered by  reproducing  the  measurements  diagram- 
matically,  though  in  this  case  they  must  be  arranged  on 
a  different  system.  In  the  accompanying  diagram,  Fig. 
2,  65  measurements  of  the  wing  of  Sterna  hirundo, 
recorded  in  the  above-mentioned  paper  of  Allen,  are 
plotted  out.  Here  each  dot  represents  one  measure- 
ment, all  the  measurements  between  10.46  and  10.55 
inches  being  placed  over  the  number  10.5,  and  so  on. 
The  mean  of  all  the  measurements  is  10.49  inches,  and 


8  THE  MEASUREMENT  OF  VARIATION. 

we  see  in  the  diagram  that  the  most  frequently  occur- 
ring measurement  is  one  of  10.5  inches.  Wing  lengths 
smaller  or  greater  than  the  mean  occur  less  and  less  fre- 
quently, in  rough  proportion  to  their  degree  of  devia- 
tion from  it,  so  that  finally,  beyond  the  extreme  devia- 
tions of  9.6  and  11.7  inches,  no  measurements  were  ob- 
served at  all. 

The  number  of  observations  here  plotted  out  is  ob- 
viously much  too  small  to  yield  at  all  a  regular 
series,  but  it  is  quite  sufficient  to  show  that  the 


•  •  •  • 


9-5  10-0  10-5  11-0  11-5 

FIG.  2. — Wing  of  Sterna  Mrundo. 

measurements  are  by  no  means  evenly  distributed 
through  the  whole  range  of  their  variation.  There  is 
a  most  conspicuous  collection  of  them,  or  heaping  up, 
in  the  region  of  the  mean  measurement.  Supposing 
the  number  of  observations  were  increased,  then 
one  would  expect  as  a  general  rule  to  get  a  more  and 
more  even  series;  in  fact,  to  get  a  fairly  accurate  idea 
as  to  the  kind  of  series  obtainable,  supposing  an  in- 
finite number  of  observations  were  made.  In  Fig.  3 
is  plotted  out  a  curve  representing  the  distribution  of 
1923  measurements  made  by  Warren  *  on  a  certain 
dimension,  viz.,  the  carapace  breadth  of  the  crab  Por- 
*Proc.  Roy.  Soc.,  vol.  Ix.  p.  225. 


I 


CO        « 

I 

I 


a 


II 

a" 


I 


C  nte 


oic 


Ve 


tica 


10  THE  MEASUREMENT  OF  VARIATION. 

tunus  depurator.  In  order  to  get  rid  as  far  as  possible 
of  the  factor  of  size,  and  obtain  a  measure  of  the  varia- 
bility apart  from  this,  each  measurement  was  calculated 
as  a  fraction  on  that  of  the  carapace  length  of  the 
crab  taken  as  1000.  The  numbers  on  the  abscissa  line 
therefore  represent  1230,  1240,  etc.,  thousandths  of 
the  total  length.  The  figures  on  the  central  ordinate 
represent  the  numbers  of  individuals  of  each  particular 
dimension.  For  instance,  one  may  gather  that  16  indi- 
viduals had  a  post-spinous  length  of  1260, 172  of  them 
one  of  12  97,  and  so  on. 

If  this  curve  be  compared  with  the  general  contour 
of  the  previous  figure,  it  will  be  seen  at  a  glance  that 
there  is  a  much  more  regular  rise  and  fall,  especially  in 
regard  to  the  extreme  measurements.  In  fact,  it  does 
not  differ  very  greatly  from  the  dotted  line  curve  upon 
which  it  is  superposed,  and  supposing  the  number  of 
observations  had  been  greater,  one  would  expect  the 
approximation  to  be  still  closer;  supposing  it  had  been 
infinitely  great,  one  would  expect  the  two  curves  to  be 
identical.  Now  this  dotted  line  is  a  probability  curve, 
or  a  diagrammatic  representation  of  the  Law  of  Fre- 
quency of  Error,  of  which  the  mathematical  expression* 
was  first  deduced  by  Gauss  at  the  beginning  of  the 
last  century.  It  would  be  out  of  place  to  attempt 
to  reproduce  its  mathematical  proof  here,  but  perhaps 
a  concrete  instance  may  help  to  bring  home  to  the  non- 
mathematical  reader  the  fact  that  variability  does  obey 

*  This  expression  is  y  =  ke  -W  «a,  or  taking  k  and  h  each  as  unity, 
y  =  — s,  where  e  is  the  base  of  Naperian  logarithms,  and  y  an  ordi- 

6  •" 

nate  erected  from  any  point  on  the  abscissa,  distant  x  from  the 
middle  ordinate. 


THE  MEASUREMENT  OF  VARIATION.  11 

the  laws  of  chance.  Supposing  a  group  of  developing 
organisms  be  taken,  of  which  the  growth  can  be  affected 
in  a  favourable  or  an  unfavourable  manner  by  their 
surroundings.  Let  us  suppose  that  there  are  twenty 
different  agencies,  each  of  which  would  produce  an 
equal,  favourable  effect  on  growth,  and  twenty  which 
would  produce  just  as  great  an  effect  in  the  opposite 
direction.  Suppose  also  that  each  organism  is  sub- 
jected to  only  half  of  these  forty  different  agencies; 
then  it  would  follow,  according  to  the  laws  of  chance, 
that  a  larger  number  of  the  organisms  would  be  acted 
upon  by  10  favourable  and  10  unfavourable  agencies, 
than  by  any  other  combination;  i.  e.,  they  would,  on  our 
hypothesis,  remain  absolutely  unaffected  in  their 
growth.  A  somewhat  smaller  number  would  be  acted 
upon  by  11  favourable  and  9  unfavourable  agencies,  or 
on  the  whole,  would  have  their  growth  slightly  in- 
creased. A  still  smaller  proportion  would  be  acted  on 
by  12  favourable  and  8  unfavourable  agencies,  or  would 
have  their  growth  rather  more  increased.  Finally  the 
number  of  organisms  acted  on  by  20  favourable  and  0 
unfavourable  agencies  would  be  extraordinarily  small, 
but  in  this  case  the  effect  on  growth  would  be  extremely 
large.  Similar  relationships,  only  in  the  reverse  direc- 
tion, would  of  course  be  found  in  those  cases  in  which 
the  number  of  unfavourable  agencies  exceeded  the 
number  of  favourable.  If  desired,  the  proportional 
numbers  of  organisms  acted  on  by  all  the  different  com- 
binations of  agencies  may  be  readily  determined  by  ex- 
panding the  binomial  (-J  +  J)  20.  It  is  found,  for  in- 
stance, that  for  each  single  time  the  organisms  are 
acted  on  by  the  whole  20  favourable  agencies,  they  are 


12          THE  MEASUREMENT  OF  VARIATION. 

acted  on  190  times  by  18  favourable  and  2  unfavour- 
able, 15,504  times  by  15  favourable  and  5  unfavour- 
able, and  no  less  than  184,756  times  by  10  favourable 
and  10  unfavourable.  Let  us  consider  that  the  organ- 
isms acted  on  by  20  favourable  and  0  unfavourable 
agencies  have  their  size  increased  by  20  per  cent.,  those 
acted  on  by  15  favourable  and  5  unfavourable  by  15  — 
5  =  10  per  cent.,  and  so  on.  If  now  these  percentage 
increments  and  decrements  be  plotted  out  at  equal  dis- 
tances on  a  base  line,  and  ordinates  corresponding  to 
the  theoretical  frequencies  erected  from  each,  then  by 
joining  these  ordinates  we  shall  obtain  a  curve  which  is 
practically  identical  in  form  with  the  dotted  line  curve 
given  in  Fig.  3;  i.  e.,  with  the  probability  curve  of  the 
law  of  frequency  of  error.  Thus,  by  a  simple  arith- 
metical method,  we  can  obtain  a  series  approximating 
more  and  more  closely  to  the  probability  curve,  the 
greater  the  number  of  times  the  expression  (J  +  J)  is 
expanded.  Expanded  20  times,  the  average  error  is 
less  than  .5  per  cent.,  and  for  a  greater  number  of  times 
it  becomes  rapidly  smaller  and  smaller. 

The  deviations  in  the  dimensions  of  organisms  are 
thus  distributed  about  their  mean  in  a  symmetrical 
manner,  in  accordance  with  the  law  of  frequency  of 
error.  This  is  true  not  of  one  or  two  characteristics 
of  an  organism,  but  probably,  in  the  majority  of  cases, 
of  nearly  all  of  them.  The  dependence  of  variation  on 
the  Laws  of  Probability  was  first  demonstrated  by  Que- 
telet  *  in  the  case  of  height  and  chest  measurements  of 
soldiers.  These  he  showed  to  group  themselves  in  ac- 
cordance with  the  ordinates  of  a  binomial  curve. 
*  "  Lettres  sur  la  theorie  des  probabilites,"  Brussels,  1846. 


THE  MEASUREMENT  OF  VARIATION.  13 

Subsequently  *  he  proved  that  a  similar  relationship 
was  true  not  only  for  the  height,  weight,  strength,  lon- 
gevity, and  other  physical  qualities  of  man,  but  also  for 
his  intellectual  and  moral  qualities,  such  as  age  at  mar- 
riage, age  of  criminals,  and  so  on.  He  considered  also 
that  these  laws  extend  to  the  whole  Animal  and  Vege- 
table Kingdoms,  though  he  did  not  give  proofs  of  this 
hypothesis. 

In  confirmation  and  extension  of  Quetelet's  results, 
the  observations  of  Mr.  Francis  Galton  f  may  be 
quoted.  These  were  made  at  the  Anthropometric 
Laboratory  of  the  International  Health  Exhibition  of 
1884,  upon  from  489  to  1788  men  and  women.  It  was 
found  that  the  variations  in  height,  span  of  arms, 
weight,  breathing  capacity,  strength  of  pull,  strength  of 
squeeze,  swiftness  of  blow,  and  keenness  of  sight  all  con- 
formed in  their  distribution  to  the  Law  of  Error.  With 
regard  to  the  lower  animals,  Professor  Weldon  £  has 
made  measurements  on  the  carapace,  post-spinous  por- 
tion of  carapace,  length  of  the  sixth  abdominal  tergum, 
and  length  of  telson,  in  the  case  of  two  to  five  local  races 
of  shrimps,  and  obtained  a  similar  result.  He  has 
also  §  made  no  less  than  eleven  different  series  of 
measurements  on  999  female  crabs  (Carcinus  mcenas) 
obtained  from  Plymouth  Sound,  and  a  similar  number 
on  999  specimens  obtained  from  the  Bay  of  Naples. 
Twenty  series  of  frequencies  of  deviation  from  the 
average  were  thereby  obtained,  and  were  found  in  every 

*  "  Anthropometrie,"  p.  257, 1870. 
f  "  Natural  Inheritance,"  p.  201. 

t  Proc.  Roy.  Soc.,  xlvii.  p.  445, 1889,  and  Proc.  Roy.  Soc.,  li.  p.  2, 
1892. 
§Proc.  Roy.  Soc.,  liv.  p.  318,  1893. 


14          THE  MEASUREMENT  OF  VARIATION. 

case  but  one  to  conform  to  that  required  by  the  Law  of 
Error.  The  single  exception  to  the  general  rule  will 
be  referred  to  in  the  next  chapter.  Again,  H.  Thomp- 
son *  made  twenty-two  different  measurements  on  1000 
adult  prawns,  and  found  the  variations  in  every  case  but 
one  to  correspond  more  or  less  accurately  with  the  law. 
E.  Warren  f  made  seven  different  measurements  on 
2300  male  crabs  (Portunus  depuraior),  obtained  from 
Plymouth,  and  found  that  the  variations  very  nearly 
corresponded  to  the  law.  Duncker  $  made  eight  series 
of  determinations  on  the  number  of  spines  and  rays  in 
the  fins  of  1900  specimens  of  the  fish  Acerina  cernua, 
and  found  that  with  one  slight  exception  the  variations 
obeyed  the  general  law.  Of  the  twelve  series  of  meas- 
urements §  made  on  1120  specimens  of  the  flounder 
(Pleuronectes  flesus),  however,  only  six  were  quite  sym- 
metrical and  in  accordance  with  the  law.  Finally  the 
author  ||  made  9850  measurements  on  the  plutei  or 
larvsB  of  a  sea-urchin,  Strongylocentrotus  lividus,  and 
found  that  the  variations  in  size  corresponded  very 
closely  indeed  with  the  law.  The  lengths  of  the  arms 
of  these  plutei  were  calculated  as  percentages  on  the 
length  of  body,  and  were  found  in  the  case  of  the  oral 
arm  lengths  to  correspond  closely  with  theory,  but  in 
the  case  of  the  anal  arm  lengths,  there  was  some  slight 
divergence. 

With  regard  to  the  variation  of  plants,  our  accurate 

*Proc.  Roy.  Soc.,  Iv.  p.  234,  1894. 

fProc.  Roy.  Soc.,  Ix.  p.  221. 

t  Biologischen  Centralblatt,  xvii.  p.  785,  1897. 

gZool.  Anzeig.,  xxiii.  p.  141. 

flPhil.  Trans.  1895,  B.  p.  613. 


THE  MEASUREMENT  OF  VARIATION.  15 

knowledge  is  derived  chiefly  from  the  work  of  Ludwig, 
De  Vries,  and  Vb'chting.  The  majority  of  variations 
hitherto  examined  have  not  been  found  to  be  at  all  ac- 
curately in  accordance  with  the  law  of  frequency  of 
error,  for  reasons  which  will  be  referred  to  later. 
However,  in  the  case  of  one  or  two  local  races  of  Torilis 
anthriscus  (hedge  parsley)  examined  by  Ludwig*,  the 
distribution  of  the  frequencies  of  the  numbers  of 
branches  in  the  main  umbels  more  or  less  conforms, 
and  the  same  is  true  for  the  numbers  of  ray  florets  in  a 
pure  race  of  Chrysanthemum  segetum  (corn  marigold) 
examined  by  De  Vries.  f  Again  H.  VochtingJ  has 
recently  examined  the  anomalies  occurring  in  61,736 
flowers  of  Linaria  spuria  (toadflax),  obtained  in  differ- 
ent years  and  from  different  sources.  He  determined 
the  proportions  of  the  various  forms  of  peloric  flowers 
and  anomalous  zygomorphic  forms,  of  flowers  of  vary- 
ing structure  and  with  various  numbers  of  spurs,  and 
came  to  the  conclusion  that  their  distribution  followed 
the  law  of  error.  For  instance,  the  numbers  of  flowers 
in  each  inflorescence  showed  the  following  variations : 

Number  of  flowers,      234  5  678 

Frequency,  1          6       283    61,060         221         9  1 

Percent.,  .0016  .0097      .459    99.153        .358    .014     .0016 

Here  we  see  that  though  more  than  99  per  cent,  of  the 
flowers  exhibited  the  normal  pentamerous  form,  yet 
the  variations  from  this  normal  are  very  evenly 
distributed  on  either  side  of  it.  The  distribution  of 
the  numbers  in  all  the  peloric  flowers  (i.  e.,  regular 

*Bot.  Centralb.,  vol.  Ixiv.  p.  40. 

f  Arch.  f.  Entwickelungsmechanik,  ii.  p.  52, 1896. 

\  Jahrb.  f.  wiss.  Bot.,  Bd.  xxxi.  p.  391,  1898. 


16 


THE  MEASUREMENT  OF  VARIATION. 


flowers,  instead  of  the  normal  irregular  ones)  was  as 
follows : 


Number  of  flowers,      234  5 

Frequency,  1          2         43         810 

Percent.,  .109     .219     4.720    88.913 


678 

52         2  1 

5.708    .219      .109 


From  these  two  series  a  very  interesting  relationship 
declares  itself,  which  may  for  convenience  be  referred 
to  here,  though  it  properly  comes  under  the  heading  of 
"correlated  variations."  Thus,  as  the  following  fig- 
ures show,  we  find  that  the  probability  of  occurrence 
of  a  peloric  flower  increases  according  to  the  amount  of 
deviation  of  the  number  of  flowers  on  a  stalk  from  the 
normal  pentamerous  form,  or  that  the  less  often  a 
particular  number  of  flowers  occurs,  the  more  fre- 
quently does  it  produce  peloric  flowers  : 

per  cent,  of  the      5      flower  form  have  peloric  flowers. 


.132 
15.19 
23.53 
22.22 
33.33 
100.00 


5 

4 
6 
7 
3 
2  and  8 


Of  English  observers,  J.  H.  Pledge  *  has  determined 
the  variations  in  the  numbers  of  petals,  stamens,  and 
carpels  in  1000  specimens  of  Ranunculus  repens  (creep- 
ing crowfoot),  the  distributions  of  all  but  the  numbers 
of  petals  agreeing  fairly  closely  with  the  probability 
integral.  For  instance,  the  numbers  of  sepals  varied 

thus: 

Sepals,  34  567 

Frequency,     1        20        959        18        2 

We   see,   therefore,    that   in    the   majority   of   the 
characteristics  of  the  various  organisms  investigated, 
*Nat.  Science,  vol.  x.  p.  323,  and  vol.  xii.  p.  179,  1898. 


THE  MEASUREMENT  OF  VARIATION.  17 

especially  those  belonging  to  the  Animal  Kingdom, 
the  variations  are  distributed  about  their  mean  in 
accordance  with  the  Law  of  Error.  It  is  scarcely 
necessary  to  point  out,  however,  that  the  actual 
range  of  the  variations  is  exceedingly  variable,  and 
that  the  general  contour  of  the  curves,  supposing 
the  results  are  expressed  in  that  way,  must  be  equally 
variable.  The  greater  the  variability  of  any  char- 
acteristic, the  more  spread  out,  or  flattened,  must  be 
the  curve  representing  the  frequencies  of  its  devia- 
tions. If,  therefore,  results  were  invariably  expressed 
in  the  form  of  curves,  and  if,  by  multiplying  each 
series  of  measurements  by  some  factor,  the  central 
ordinate  were  always  brought  to  the  same  height, 
then  it  would  follow  that  the  variability  of  each  char- 
acteristic would  be  accurately  represented  by  the  ex- 
tent of  spread  of  the  curve.  In  order  to  obtain  an  in- 
dex of  the  variability  of  any  characteristic,  we  must 
accordingly  adopt  some  convenient  method  of  deter- 
mining the  degree  of  spread  of  its  curve.  One  of  the 
simplest  of  these  methods,  and  one  widely  employed  by 
English  statisticians,  is  to  determine  the  so-called 
Probable  Error.  The  meaning  of  this  term  is  best 
explained  by  reference  to  the  accompanying  diagram  of 
a  curve  of  frequency  of  error.  The  ordinate  drawn 
through  the  middle  of  the  curve  is  spoken  of  by  Mr. 
Galton  as  the  Median,  and  is  denoted  by  the  symbol  M. 
In  symmetrical  curves  it  is  identical  with  the  ordinary 
arithmetic  mean  or  average,  and  in  this  sense  is  called 
the  Centroid  Vertical.  It  is  the  middle  value  of  the 
whole  series  of  observations,  which  are  symmetrically 
distributed  on  each  side  of  it.  That  is  to  say,  50  per 


18 


THE  MEASUEEMENT  OF  VARIATION. 


cent,  of  all  the  observations  fall  below  it  in  magnitude, 
and  50  per  cent,  above  it.  The  actual  number  of  ob- 
servations made  is  obviously  represented  by  the  area  of 
the  figure  enclosed  by  the  curve  and  the  abscissa  line, 
or  the  so-called  "  polygon  of  variation."  The  area  to 
the  left  of  the  median  corresponds  to  the  half  of  the 
observations  of  less  magnitude  than  the  average,  and 


SAQ,         M  Q,A'S' 

FIG.  4. — Normal  Curve  of  Error. 

that  to  the  right,  of  those  of  greater  magnitude.  Now 
let  two  other  ordinates,  Ql  and  Q3,  be  erected  so  as  to 
divide  each  of  these  areas  into  equal  halves.  We 
now  have  four  areas  representing  four  numerically 
equal  groups;  i.  e.,  all  the  observations  of  small  magni- 
tude from  0  to  25  per  cent,  of  the  whole;  those  of 
greater  magnitude,  from  25  per  cent,  to  50  per  cent,  of 
the  whole ;  those  of  greater  magnitude  than  the  average, 
representing  50  per  cent,  to  75  per  cent.,  and  finally 
those  of  greatest  magnitude,  representing  the  remain- 
ing 25  per  cent.  Half  of  all  the  observations  there- 
fore exceed  the  limits  of  these  ordinates  Qi  and  Q3,  and 
half  of  them  fall  between  or  within  them;  so  the  dis- 
tance on  the  abscissa  line  from  M  to  Ql  or  M  to  Q3,  is 


THE  MEASUREMENT  OF  VARIATION. 


19 


called  the  "  Probable  Error  "  of  variation.  In  a  per- 
fectly normal  curve,  these  values  are  equal  in  value 
and  opposite  in  sign,  but  as  no  experimental  result  is 
perfect,  they  usually  differ  slightly  in  amount.  A 
mean  between  the  two  is  therefore  taken,  and  this  is 
denoted  by  the  symbol  Q. 

For  the  practical  determination  of  the  probable 
error,  however,  it  is  quite  unnecessary  to  plot  out  the 
results  in  the  form  of  a  curve.  The  method  adopted 
is  best  illustrated  by  a  concrete  instance.  In  the  ac- 
companying table  are  given  the  results  obtained  by 
Mr.  Galton  *  for  the  strength  of  pull,  as  of  an  archer 
with  a  bow,  of  519  males,  aged  23  to  26 : 


PERCENTAGES. 

STRENGTH  OP  PULL. 

OBSERVED. 

NUMBER  OP  CASES 

SUMS  FROM  BE- 

OBSERVED. 

GINNING. 

Under  50  Ibs. 

10 

2 

2 

60 

42 

8 

10 

70 

140 

27 

37 

80 

168 

33 

70 

90 

113 

21 

91 

100 

22 

4 

95 

Above  100 

24 

5 

100 

Here  we  see  that  the  numbers  of  actual  cases  in  each 
group  are  given  in  the  second  column,  and  that  they  are 
calculated  as  percentages  in  the  third  column.  They 
are  summed  from  the  beginning  in  the  fourth  column, 
and  we  thereby  gather  that  whilst  only  37  per  cent,  of 
all  the  men  had  a  strength  of  pull  under  70  Ibs.,  70  per 
cent,  of  them  had  one  under  80  Ibs.  It  can  be  calcu- 
*"  Natural  Inheritance,"  p.  199. 


20          THE  MEASUREMENT  OF  VARIATION. 

lated,  therefore,  that  50  per  cent,  of  them  had  a 
strength  of  pull  under  74  Ibs.,  or,  in  Mr.  Galton's  nota- 
tion, the  strength  of  pull  at  Grade  50°,  was  under  74 
Ibs.  This,  then,  is  the  average  strength  of  pull,  or  M , 
of  the  whole  group.  Fifty  per  cent,  of  the  men  pulled 
less  than  this  amount,  and  50  per  cent,  of  them  more. 
Similarly,  also,  one  can  calculate  that  25  per  cent,  of  the 
men  would  have  a  pull  of  less  than  66  Ibs.,  and  75  per 
cent,  one  of  greater  amount,  whilst  75  per  cent,  would 
have  one  of  less  than  82  Ibs.,  and  25  per  cent,  one  of 
greater.  That  is  to  say,  the  strengths  of  pull  at  Grades 
25°  and  75°  were  respectively  66  and  82  Ibs.  The  prob- 
able error  of  variation  in  pull,  or  QJ?  is  therefore  equal 
to  74  —  66  =  8  Ibs.,  and  also  to  Q3,  or  82  —  74  =  8 

Ibs.,  whilst  the  mean  value  which  is  always  in  practice 

8-4-8 
adopted  as  the  probable  error,  or  Q,  is  — ~ —  =  8  Ibs. 

This  probable  error  is  10.8  per  cent,  on  the  magnitude 
of  the  average  strength  of  pull,  and  this  value  accu- 
rately represents  the  variability  of  this  group  of  men  in 
respect  of  this  particular  characteristic.  Supposing  an- 
other group  were  found  to  have  a  probable  error  of  only 
5.4  per  cent,  on  the  magnitude  of  the  average,  then 
one  would  be  justified  in  saying  that  th^ir  variability, 
or  range  of  variation,  was  only  half  as  great;  or  if  it 
had  been  21.6  per  cent.,  then  twice  as  great. 

This  relative  probable  error  is  therefore  a  con- 
venient index  of  variability  of  any  characteristic. 
A  few  examples  may  be  quoted  in  order  to  give 
an  idea  as  to  its  range.  From  the  anthropometric  data 
obtained  by  Mr.  Galton,  it  is  calculated  that  the  index 
was  2.50  per  cent,  for  man's  stature,  and  2.52  per  cent. 


THE  MEASUREMENT  OF  VARIATION.  21 

for  woman's;  2.92  per  cent,  for  the  span  of  arms  of 
both  man  and  woman;  but  no  less  than  6.89  per  cent, 
for  man's  weight,  and  8.89  per  cent,  for  woman's.  That 
is  to  say,  weight  is  more  than  twice  as  variable  as  the 
other  two  characteristics.  In  most  of  Weldon's  shrimp 
and  crab  measurements  the  amount  of  variability  was 
considerably  smaller,  but  this  was  partly  due  to  the  fact 
that  the  element  of  size  was  largely  excluded  by  first 
of  all  calculating  all  measurements  as  thousandths  of 
the  body  and  carapace  lengths  respectively.  In  1000 
shrimps  from  Plymouth,  the  total  carapace  length  had 
a  relative  probable  error  of  only  1.82  per  cent.,  the 
post-spinous  carapace  length  one  of  1.97  per  cent.,  the 
sixth  abdominal  tergum  one  of  1.93  per  cent.,  and  the 
telson  one  of  2.36  per  cent.  In  999  crabs  obtained 
from  Naples,  the  value  was  only  1.07  per  cent,  for  the 
total  breadth  of  carapace,  and  from  1  to  2  per  cent, 
for  several  of  the  other  measurements  made,  but  in  the 
carpopodite  of  the  right  chela  it  rose  to  3.63  per  cent., 
and  in  the  proximal  portion  of  the  chela  to  no  less  than 
5.77  per  cent.  These  last  were,  however,  quite  ex- 
ceptionally large  degrees  of  variation.  Still,  in  the  sea- 
urchin  larvae  measured  by  the  author,  the  variability 
was  found  to  be  greater  than  in  any  of  these  instances 
recorded  in  the  higher  animals,  it  being  6.1  per  cent, 
for  the  body  length,  9.4  per  cent,  for  the  oral  arm 
length,  and  11.3  per  cent,  for  the  anal  arm  length. 

We  have  seen  that  the  degree  of  correspondence  of 
the  variations  in  any  characteristic  with  the  law  of 
error  can  be  determined  by  plotting  out  the  results  in 
the  form  of  a  curve,  but  this  is  clearly  a  somewhat 
laborious  process.  It  is  much  simpler  and  more  con- 


22 


THE  MEASUREMENT  OF  VARIATION. 


venient  to  compare  the  experimental  and  theoretical 
values  directly  by  a  numerical  method.  This  is  done 
by  extending  the  method  of  grades  referred  to  above. 
In  addition  to  determining  the  magnitude  of  the  char- 
acteristics at  grades  25°,  50°,  and  75°,  one  determines  it 
also  at  grades  5°,  10°,  20°,  30°,  and  so  on,  or  determines 
the  values  having  respectively  5,  10,  20,  30  per  cent., 
etc.,  of  all  the  measurements  below  them  in  magnitude, 
and  95,  90,  80,  70  per  cent.,  etc.,  above  them  in  magni- 
tude. Let  the  median,  or  value  at  grade  50°,  be  now 
subtracted  from  the  values  at  all  the  other  grades,  and 
the  numbers  so  obtained  be  divided  by  the  probable 

error,  or  — 1—= — !.    We  then  obtain  a  series  of  values  at 

the  various  grades,  in  terms  of  the  probable  error  taken 
as  unity;  so  that,  whatever  had  been  the  magnitude  of 
the  median,  and  of  the  probable  error,  the  values  are 
now  directly  comparable  with  the  theoretical  values  cal- 
culated from  the  probability  integral.  These  theoretical 
values  are  given  in  the  first  line  of  the  subjoined  table : 


GRADE. 

5° 

10° 

20° 

25° 

30° 

40° 

50° 

60° 

70° 

75° 

80° 

90° 

95° 

THEORETICAL  VALUES 

2.44 

1.90 

1.25 

1.00 

.78 

.38 

.00 

.38 

.78 

1.00 

1.25 

1.90 

2.44 

9443  anthropometric 
measurements. 

2.44 

1.87 

1.84 

1.00 

.77 

.40 

,00 

.38 

.75 

.98 

1.21 

1.92 

2.47 

400  shrimp  carapace 

length  measurements 
9850  measurements  of 

2.42 

1.86 

1.22 

1.00 

.79 

.39 

.00 

.32 

.71 

1.00 

1.28 

2.10 

2.63 

sea-urchin  larvae,  . 

2.51 

1.92 

1.25 

1.01 

.79 

.38 

.00 

.37 

.77 

.99 

1.24 

1.90 

2.46 

Beneath  them  are  given  the  means  of  the  values  ob- 
tained by  Mr.  Galton  for  18  different  series  of  measure- 
ments on  men  and  women,  the  total  number  of  observa- 
tions made  being  9443.  In  the  individual  series  the 


THE  MEASUREMENT  OF  VARIATION.  23 

deviations  from  the  theoretical  values  were  of  course 
greater,  but  these  differences  almost  completely  neu> 
tralise  each  other  in  the  general  mean.  Indeed  the 
correspondence  is  extraordinarily  close?  considering  the 
very  mixed  nature  of  the  faculties  measured,  viz.,  three 
linear  measurements,  one  of  weight,  one  of  capacity, 
two  of  strength,  one  of  vision,  and  one  of  swiftness. 
The  next  series  of  values  is  that  obtained  by  Professor 
"Weldon  for  400  shrimps.  It  is  given  to  show  that  in 
the  case  of  a  comparatively  small  number  of  observa- 
tions, the  correspondence  between  fact  and  theory  may 
be  very  close  indeed.  Finally,  in  the  bottom  line  of  the 
table  are  given  the  values  obtained  by  the  author  for 
9850  measurements  on  the  body  length  of  sea-urchin 
larvae.  Here  the  correspondence  is  closer  even  than  in 
the  anthropometric  measurements,  the  average  differ- 
ence being  only  .014,  as  against  .0175. 

In  order  to  express  the  variability  of  a  characteristic, 
we  are  by  no  means  limited  to  the  method  of  determin- 
ing the  probable  error.  A  much  older  method  is  that  of 
the  arithmetic  mean  error,  or  average  deviation.  This 
value  consists  of  the  mean  of  all  the  deviations,  both 
positive  and  negative,  from  the  general  mean.  For  in- 
stance, to  determine  the  arithmetic  mean  error  of  the 
following  series  of  16 — 

7,  8,  8,  9,  9,  9,  10,  10,  10,  10,  11,  11,  11,  12,  12,  13. 
•figures,  one  calculates  the  general   mean,  viz.,  10,  and 
determines  their  deviations  from  it.  These  are 

3,  2,  2,  1,  1,  1,  0,  0,  0,  0,  1,  1,  1,  2,  2,  3. 
Added  together  these  equal  20,  so  that  the  arithmetic 

20 
mean  error  is        =  1.25.     In  practice,  it  is  sometimes 


24  THE  MEASUREMENT  OF  VARIATION. 

simpler  to  separate  all  the  numbers  into  two  groups,  one 
containing  all  the  values  greater  than  the  general  mean, 
and  the  other  all  those  less  than  the  mean.  Then  the 
arithmetic  mean  error  is  half  the  difference  between  the 
mean  of  each  group.  When,  as  in  the  present  instance, 
several  of  the  values  are  identical  with  the  mean,  half 
of  them  must  be  put  in  each  group.  The  mean  of  one 

70 
group  is   now  —  =  8.75,   and   of    the    other    group 

90 

•g-  =  11.25,      and     the     arithmetic     mean     error    is 

11.25-8.75 


2 

This  method  of  estimating  variability  has  frequently 
been  employed  in  recent  times,  especially  in  America. 
Thus  Minot  *  used  it  for  comparing  the  variability  of 
guinea-pigs  at  various  periods  of  their  growth.  Brew- 
ster  f  used  it  for  calculating  the  amount  of  varia- 
tion in  a  number  of  head,  face,  and  limb  measurements 
which  were  made  by  Weisbach  J  on  individuals  of  23 
different  races  of  men.  In  the  general  mean  are  in- 
cluded the  measurements  of  195  individuals,  represent- 
ing 20  different  races.  The  following  are  some  of  the 
mean  values  for  the  arithmetical  mean  error,  calculated 
as  percentages  on  the  mean  size : 

Nose  length,  9.49  per  cent.     Head  length,  2.44  per  cent. 

"    breadth,        7.57         *  "    breadth,          2.78 

"    height,          15.2         '  Upper  arm  length,  6.50 

Forehead  height,    10.4         '  Forearm  length,      3.85 

Under  jaw  length,  4.81         '  Upper  leg  length,    5.00 

Mouth  breadth,      5.18         *  Lower        "  5.04 

Foot  length,  5.92 

*  J.  Physiol.,  xii.  p.  138,  1891. 
fProc.  Amer.  Acad.  Arts  Sci.,  xxxii.  p.  268,  1897. 
j  Zeitschrift  f.  Ethnologic,  Bd.  ix.    Supplement,  1878. 


THE  MEASUREMENT  OF  VARIATION.  25 

Here  we  see  that  most  of  the  face  measurements  are 
far  more  variable  than  most  of  the  head  and  limb 
measurements;  that  of  the  nose  height,  for  instance, 
being  six  times  as  great  as  that  of  the  head  length. 
The  high  value  which  is  universally  accorded  to  facial 
proportions  as  a  means  of  personal  identification  thus 
receives  its  numerical  justification. 

On  comparing  the  variability  of  the  measurements 
in  the  individuals  of  eight  different  races,  it  was  found 
to  be  more  or  less  the  same  in  each  case.  If  the  nose 
of  a  Jew  is  a  very  variable  organ,  so  is  that  of  a  Slav,  a 
Magyar,  or  a  Chinaman. 

Davenport  and  Bullard  *  used  the  method  of  arith- 
metic mean  error  in  the  4000  enumerations  which  they 
made  of  the  Mullerian  glands  in  the  forelegs  of  swine. 
These  glands  vary  in  number  from  0  to  10,  the  average 
being  3.53.  The  arithmetic  mean  error  was  1.41  in 
male  swine,  and  1.38  in  female  swine,  or  the  variability 
was  2.5  per  cent,  greater  in  the  one  case  than  the  other. 
Again  Garstang  f  has  used  it  to  estimate  the  variability 
of  various  local  races  of  the  mackerel. 

There  is  still  another  method  of  estimating  varia- 
bility, which  is  more  accurate  than  either  of  the  two 
mentioned,  but  which  until  recently  has  not  been  used 
so  frequently  as  they  were,  because  of  the  labour  of  ap- 
plying it.  This  is  the  method  of  Mean  or  Least  Squares. 
One  determines  the  deviations  from  the  average  in  the 
same  way  as  for  the  arithmetic  mean  error,  but  then 
squares  each  of  them,  takes  the  sum  of  these  squares, 


*Proc.  Amer.  Acad.  Arts  Sci.,  xxxii.  p.  87,  1896. 
f  Jour.  Marine  Biol.  Asso.,  vol.  v.  p.  235,  1898. 


26          THE  MEASUREMENT  OP  VARIATION. 

divides  by  the  number  of  observations,  and  takes  the 
square  root  of  the  quotient.  Thus: 

€  (or  ff)  = 

Where  n  =  number  of  observations,  and  v  =  a  devia- 
tion from  the  average.  For  instance,  to  determine  the 
variability  of  the  following  series,  representing  the 
frequencies  of  the  numbers  of  veins  in  26  leaves  col- 
lected from  different  parts  of  a  beech  tree,*  we  find  the 

Number  of  veins,      15        16        17        18        19        20 
Frequencies,  147941 

mean  (IT. 5),  determine  the  deviations  from  it  in  each 
direction,  and  square  them.  Then  the  variability  will 
be  represented  by  the  square  root  of  the  following  ex- 
pression: 

(2.5)'  X  1  +  (1.5)a  X  4+(.5)»  X  7  +  (.5)'  X  9  +  (1.5)'X  4  +(2.5)*X  1 

i.  e.,  by  1.15. 

This  index  of  variability,  or  "  Error  of  Mean  Square," 
is  termed  by  Professor  Pearson  the  "  Standard  Devia- 
tion," or  #,  and  its  percentage  ratio  on  the  mean  the 
"  Coefficient  of  Variation."  It  has  been  made  use  of 
by  Warren  in  the  crab  measurements  already  referred 
to,  and  also  in  a  very  elaborate  research  f  on  the 
variability  of  the  skeleton  of  the  Naquada  race,  a 
people  that  existed  in  Egypt  about  3500  B.  c.  It 
has  also  been  employed  by  Weldon,  whilst  Pearson  al- 
most invariably  adopts  it.  Duncker  $  has  expressed  his 

*  Vide  K.  Pearson's  "  Grammar  of  Science,"  p.  382, 

fPhil.  Trans.  1898,  B.  p.  135. 

\  Biol.  Centralblatt,  xvii.  p.  785,  1897. 


THE  MEASUREMENT  OF  VARIATION.  27 

results  on  the  variability  of  the  fin  rays  of  certain  fishes 
in  terms  both  of  the  mean  error  and  the  error  of  mean 
square.  Again,  from  data  obtained  by  Petersen,  Bum- 
pus,  Weldon,  and  himself,  Duncker  *  has  calculated 
the  error  of  mean  square,  and  obtained  the  following 
values  for  the  number  of  fin  rays  in  certain  fishes  : 


DORSAL         FIN  ANAL  TIN 

Me  Me 

Pleuronectes  flesus,  Baltic,  39.46    1.4838 

North  Sea,  41.56    1.7739 

"       Plymouth,         61.72    2.3895  43.61    1.6026 

americanus,  65.06    2.4467  48.62    1.8188 

Wtombus  maximus,  62.98    2.2533  45.86    1.6792 

And  the  following  for  the  number  of  rostral  teeth: 


DORSAL          FIN 
M.  e 


Palcsmonetes  varians,  4.3137    .8627  1.6948    .4799 

iris,  8.2819    .8145  2.9781     .4477 


These  results  show  that  though  the  average  values  of  a 
character  may  differ  considerably  even  in  the  local  races 
of  the  same  species,  yet  the  indices  of  variability 
may  remain  fairly  constant,  not  only  in  these,  but  also 
in  different  species,  and  perhaps  even  in  different 
genera  and  families.  Thus  the  two  species  of  Palcz- 
monetes  vary  by  respectively  92.0  per  cent,  and  75.7 
per  cent,  in  the  number  of  rostral  teeth  in  their  dorsal 
and  anal  fins,  but  by  only  5.9  per  cent,  and  7.2  per  cent, 
in  their  indices  of  variability.  Arguing  from  these 
data,  Duncker  f  concludes  that  one  has  no  right  to  ac- 
cept the  "  coefficient  of  variation  "  of  an  organ  as  the 

*Nat.  Science,  xv.  p.  328,  1899. 
f  Amer.  Nat.,  xxxiv.  p.  621, 1900. 


28  THE  MEASUREMENT  OF  VARIATION. 

absolute  measure  of  its  variability,  as  Verschaeffelt,* 
Brewster,f  and  others  have  done.  He  thinks  that  the 
indices  of  variability  alone  may  be  of  morphological 
significance,  for  in  this  case,  at  least,  they  are  obviously 
independent  of  the  mean  values  of  the  characters. 

How  far  Duncker's  view  is  to  be  accepted  can  only 
be  determined  by  further  enquiry.  Doubtless  it  will 
be  found  to  hold  good  occasionally,  but  I  think  that  the 
great  weight  of  evidence  at  present  available,  especially 
as  regards  measurements  of  size  and  shape,  rather  than 
those  of  numbers  of  organs,  is  in  favour  of  the  alterna- 
tive hypothesis. 

The  three  indices  of  variability  above  referred  to  are 
by  no  means  numerically  equivalent.  They  bear  the 
following  relations  to  each  other: 

Probable  error,  1.000      Corresponding  grades,  25°. 0,  75°.0 

Arithmetic  mean  error,  1.183  "  "       21°. 2,  78°. 8 

Error  of  mean  square,     1.483  "  "       16°.0,  84°.0 

Thus  the  error  of  mean  square  is  nearly  half  as  large 
again  as  the  probable  error,  and  therefore  includes  a 
proportionately  larger  percentage  of  the  deviations 
from  the  mean  within  its  limits.  On  the  frequency 
curve  given  a  few  pages  back  are  drawn  dotted  line 
ordinates  A,  A'  and  S,  S',  which  enclose  areas  of  the 
variation  polygon  corresponding  to  these  "  mean  error  " 
and  "  error  of  mean  square  "  indices  of  variability. 
The  "  probable  error  "  index  is  in  some  ways  the  most 
convenient  of  the  three,  as  it  is  the  smallest,  and  in- 
cludes within  its  limits  just  half  of  all  the  variants.  As 
the  error  of  mean  square  is  held  to  be  a  more  accurate 

*Ber.  d.  deutsch.  bot.  Ges.,  xii.  p.  350. 
f  Loc.  cit. 


THE  MEASUREMENT  OF  VARIATION.  29 

method  of  estimating  variability,  however,  the  plan  is 
sometimes  adopted  of  determining  this  index,  and  then 
reducing  it  to  terms  of  probable  error  by  multiplying 
by  .6745.  Similarly  an  arithmetic  mean  error  may 
be  reduced  to  terms  of  probable  error  by  multiplying  by 
.8453. 

It  will  have  been  noticed  that  in  the  series  of  meas- 
urements from  time  to  time  referred  to,  a  few  excep- 
tions to  the  general  law  of  distribution  of  variations 
were  mentioned.  In  these  cases  the  variations  were 
not  distributed  evenly  about  the  middle  ordinate,  but 
the  curve  of  distribution  was  asymmetrical,  or  skew. 
Such  series  as  these  are  by  no  means  uncommon,  espe- 
cially in  the  case  of  plant  statistics.  For  instance,  De 
Vries  *  found  that  the  number  of  petals  in  the  butter- 
cup varied  between  5  and  10,  the  frequency  of  distribu- 
tion being  as  follows : 

Number  of  petals,  5         6         7       8       9        10     11 

Frequency  observed,   133       55       23       7       2          20 
Theory,  136.9    48.5    22.6    9.6    3.4      .8      .2 

Here  flowers  with  the  smallest  number  of  petals  occur 
the  most,  and  those  with  the  largest  number  the  least, 
frequently.  The  values  marked  "  Theory "  in  this 
and  the  next  series  will  be  referred  to  later. 

Again  De  Yries  cultivated  a  variety  of  clover  in  which 
the  axis  is  very  frequently  prolonged  beyond  the  head 
of  the  flower,  and  bears  from  one  to  ten  blossoms.  The 
following  were  the  frequencies  of  occurrence  of  flowers 
with  none  of  these  blossoms,  or  with  various  numbers 
of  them : 

*  Ber.  d.  deutschen  bot.  Gesellschaft,  xii.  p.  203,  1894. 


30  THE  MEASUREMENT  OF  VARIATION. 

High  blossoms,  01  23456789  10 
Frequency  obsd.  325  83  66  51  36  36  18  7611 
Theory,  303.2  106.1  70.0  49.3  35.2  24.9  17.1  11.0  6.3  2.8  .5 

J.  H.  Pledge  *  observed  the  following  frequencies  in 
the  numbers  of  petals  in  Ranunculus  repens : 

Number  of  petals,  4  5  6  7  8  9  10  11  12  13 
Frequency,  8  706  145  72  38  15  7  7  1  1 

Again  E.  T.  Browne  f  found  the  following  variations 
in  the  number  of  tentaculocysts  in  the  ephyra  and  adult 
forms  of  the  medusa  Aurelia  aurita : 

Tentaculocysts,  45678  9  10  11  12  13  14  15 
Percentage  in 

1136  ephyrse,  0  .09  .5  3.0  79.1  6.7  5.4  3.1  1.4  .2  .09  0 
Percentage  in  3000 

adult  Aurelia,    .1    .1.74.178.9    6.3    4.8    3.0    1.4    .4    .1    .1 

In  each  case  the  normal  eight  tentaculocyst  form  com- 
prised nearly  four-fifths  of  the  whole,  but  the  great 
majority  of  the  abnormal  forms  possessed  more  than 
eight  tentaculocysts,  only  3.6  to  5.0  per  cent,  of  them 
having  less. 

Now  the  distribution  of  frequencies  in  these  and 
somewhat  similar  asymmetric  series  obviously  occurs  ac- 
cording to  some  orderly  plan,  but  can  a  mathematical 
expression  be  obtained  to  represent  them?  This  had 
been  found  impossible  till  within  the  last  few  years, 
when  Professor  Pearson  $  took  up  the  subject,  and 
showed  that  such  series,  if  composed  of  homogeneous 
material,  could  often  be  fitted  most  exactly  with  curves 
calculated  in  accordance  with  a  single  generalised 

*Nat.  Science,  vol.  xii.  p.  179,  1898. 

fQ.  J.  Microsc.  Sci.,  vol.  xxxvii.  p.  245,   1895,  and  Biometrika, 
I.  p.  90,  1901. 
JPhil.  Trans.  1895,  A.  p.  343, 


THE  MEASUREMENT  OF  VARIATION.  31 

mathematical  expression.  We  saw  a  few  pages  back 
that  an  expansion  of  the  binomial  (^  +  ^)  for  20  or 
more  times  gave  a  series  of  values  which  differed  very 
slightly  in  their  frequencies  from  that  required  by  the 
Law  of  Error.  Supposing,  now,  a  binomial  in  which  the 
two  terms  are  unequal  is  expanded,  then  obviously  an 
asymmetrical  series  of  values  is  obtained.  For  in- 
stance, instead  of  (-J  +  J)  let  (f  +  i)  or  (£  +,  £)  be 
expanded,  and  series  are  obtained  of  which  the  diagram- 
matic representations  are  given  in  the  two  curves  to 
the  left  of  the  accompanying  figure.  The  symmetrical 
curve  represents  the  expansion  of  (i  +  i),  the  expres- 
sion being  in  each  case  expanded  ten  times.  The  areas 
enclosed  between  each  of  these  curves  and  the  base  line, 
or  the  so-called  polygons  of  variation,  are  obviously  of 
exactly  equal  extent,  in  that  the  sum  of  the  two  terms 
expanded  is  in  each  case  equal  to  unity. 

It  follows,  therefore,  that  these  asymmetrical  series 
can  be  represented  by  the  expansion  of  the  expression 
(P  +  (?)"•*  Supposing  that  n  is  infinitely  large,  then 
curves  representing  the  expansion  would  stretch  out  to 
an  unlimited  extent  in  each  direction,  and  though  con- 
stantly approaching  nearer  and  nearer  to  the  abscissa, 
would  never  touch  it.  Supposing  n  is  some  finite  num- 
ber, as  20  or  40,  then  obviously  the  series  is  finite  also, 
and  its  curve  is  limited  in  extent.  If  the  two  terms  of 
the  binomial  are  unequal,  then  the  curve  approaches  the 

*  The  algebraical  expansion  of  this  expression  is: 
(p  +  q)  n  = 


-{•ngn  - 


32 


THE  MEASUREMENT  OF  VARIATION. 


abscissa  much  more  rapidly  on  one  side  than  on  the 
other,  and  so,  for  practical  purposes,  by  taking  various 
values  for  py  q,  and  n,  we  can  represent  series  of  the 


1         284          ff        6          7          8          9         10        11 
FIG.  5.— Types  of  binomial  curves.     (After  Duncker.) 


12 


following  five  types  by  means  of  the  above  generalised 
expression: 

I.  Asymmetrical  curves  limited  on  both  sides. 
II.  Symmetrical 

III.  Asymmetrical      "          "        "  one  side,  unlimited  on  the  other. 

IV.  Asymmetrical  curves,  unlimited  on  both  sides. 
V.  Symmetrical 

The  normal  curve  of  error  belongs  to  this  last  type. 
Pearson  has  also  pointed  out  that  the  abnormal  fre- 
quency curves  which  cannot  be  represented  by  a  point- 


THE  MEASUREMENT  OF  VARIATION.  33 

binomial  may  be  the  resultant  of  two  or  more  normal 
curves,  which  differ  in  the  position  of  their  axes,  or 
their  areas,  or  their  degree  of  spread,  or  in  all  three  of 
these  respects.* 

To  return  to  the  curves  in  Fig.  5,  we  see  that  the 
centroid  vertical  of  the  symmetrical  curve  corresponds 
to  the  summit  of  the  curve,  or  is  identical  with  the 
maximum  ordinate  or  mode,  as  it  is  sometimes  called. 
In  the  asymmetrical  curves,  however,  this  is  not  the 
case,  but  the  more  asymmetrical  the  curve,  the  greater 
is  the  distance  between  the  two.  The  ratio  between 
this  distance  and  the  index  of  variability  adopted  (such 
as  the  error  of  mean  square),  gives  a  convenient  "  index 
of  asymmetry  "  of  the  curve.  It  is  to  be  noted  also 
that  in  asymmetrical  curves  the  median,  or  middle 
value  of  the  whole  series,  such  that  50  per  cent,  of  the 
values  are  below  it  in  magnitude  and  50  per  cent,  above 
it,  no  longer  coincides  with  the  arithmetic  mean.  It 
lies  somewhere  between  the  centroid  vertical  and  the 
maximum  ordinate. 

As  to  the  practical  application  of  this  method  of  fit- 
ting series  of  variation  frequencies  with  curves,  Pro- 
fessor Pearson  gives  numerous  instances  in  the  above 
cited  memoir.  Fig.  6  will  serve  to  afford  some  idea  as 
to  the  types  of  frequency  curves  actually  met  with  in 
practical  statistics.  Type  <*  represents  the  above-men- 
tioned series  of  frequencies  which  De  Yries  obtained  for 
the  petals  of  buttercups,  and  high  blossoms  of  clover. 
It  also  represents  infantile  mortality  statistics.  Type  ft 
represents  the  relation  of  scarlet  fever  and  diphtheria 
mortality  to  age;  type  y  that  of  scarlet  fever  and 
*Proc.  Roy.  Soc.,  liv.  p.  329. 


THE  MEASUREMENT  OF  VARIATION. 


typhus  fever  cases  to  age;  type  d  that  of  typhoid  fever 
cases  to  age,  and  also  senile  mortality  statistics.  Fi- 
nally, type  s  represents  various  slight  degrees  of  skew- 
ness  which  are  frequently  found  to  occur  even  in  anthro- 


a 


FIG.  6. — Types  of  Skew  curves. 

pometric  and  other  series  which  had  previously  been 
thought  to  be  quite  symmetrical.  Most  of  the  series 
of  deviation  frequencies  obtained  by  Warren  *  for  vari- 
ous crab  measurements  were  found  by  him  to  be  better 
fitted  by  skew  curves  than  by  absolutely  symmetrical 
ones. 

Again,  of  the  twelve  series  of  measurements  made  by 
Duncker  f  on  the  Flounder  (Pleuronectes  flesus),  the  six 
which  showed  regular  variations  (number  of  rays  in 
dorsal,  anal,  and  pectoral  fins)  were  found  to  give  very 

*Proc.  Roy.  Soc.,  Ix.  p.  221. 

f  Wissenschaf tliche  Meeresuntersuchungen  aus  der  biologischeu 
Anstalt  auf  Helgoland,  Bd.  iii.  p.  339,  1900. 


THE  MEASUREMENT  OF  VARIATION.  35 

slightly  asymmetrical  curves  of  variation,  the  varia- 
bility of  the  bilateral  homologous  measurements  being 
always,  with  one  exception,  slightly  higher  on  the  blind 
than  on  the  eye  side  of  the  fish.  Still  the  departure  of 
some  of  the  curves  from  the  normal  Gaussian  curve  was 
only  very  slight.  The  degree  of  difference  between  the 
actual  frequencies  obtained  in  any  series  of  measure- 
ments and  the  theoretical  frequencies  calculated  from 
the  type  of  curve  found  to  show  the  closest  agreement 
with  them,  is  best  represented  by  determining  the  per- 
centage difference  of  each  actual  frequency  from  each 
theoretical  frequency,  and  then  taking  an  (arithmetical) 
average  of  the  whole.  This  average  percentage  differ- 
ence of  theoretical  and  actual  values  may  be  repre- 
sented by  the  sign  A  .  In  the  case  of  four  of  the  above 
series  of  measurements,  the  A  was  only  2.20  per  cent, 
when  the  measurements  were  compared  with  a  normal 
Gaussian  curve,  and  1.82  per  cent,  when  compared  with 
a  slightly  asymmetrical  curve  (Pearson's  Type  IV). 
The  "  fit "  was  therefore  better  with  the  asymmetrical 
curve,  but  only  very  slightly  so.  It  is  open  to  question, 
therefore,  whether  any  practical  object  is  gained  by 
estimating  exceedingly  slight  degrees  of  asymmetry  in 
series  of  measurements.  The  labour  of  so  doing  is  very 
considerable,  and  it  may  well  be  doubted  whether  it 
would  not  be  more  profitably  employed  in  making  more 
extended  series  of  observations,  and  subjecting  them  to 
less  rigid  examination.  Some  recent  observations  of 
Miss  Hefferan  *  are  instructive  in  this  connection. 
These  were  made  upon  the  frequency  of  distribution  of 
the  numbers  of  teeth  on  the  jaw  of  an  annelid,  Nereis 
*  Biol.  Bulletin,  vol.  ii.  p.  129, 1900. 


36  THE  MEASUREMENT  OF  VARIATION. 

limbata.  Four  hundred  individuals  were  examined, 
and  it  was  found  that,  as  regards  the  distribution  of  the 
total  number  of  teeth,  the  left  total  fell  into  a  curve  of 
Pearson's  Type  I,  whilst  the  right  total  was  of  Type  IV. 
However,  by  dropping  out  a  single  individual  from  the 
series,  it  was  found  that  the  curve  was  thrown  from 
Type  IV  to  Type  I.  As  Miss  Hefferan  points  out,  this 
raises  a  serious  question  as  to  the  biological  importance 
of  the  distinction  between  Pearson's  Type  I  and  Type 
IV.* 

*  Should  any  further  information  regarding  these  asymmetrical 
curves  be  desired,  the  reader  should  consult  Professor  Pearson's 
memoir  on  the  subject,  or,  if  he  is  not  a  mathematician,  then  a 
recently  published  book  by  Davenport  on  "  Statistical  Methods,"* 
and  also  a  paper  by  Duncker  on   "Die  Methode  der  Variations- 
statistik  "  f  may  be  referred  to.    Both  of  these  are  said  to  be  written 
specially  for  biologists.     I  must  mention  my  special  indebtedness  to 
Duncker's  paper,  which  has  been  drawn  upon  freely  in  writing  the 
last  few  pages  of  the  present  chapter. 

*  New  York  and  London,  1899. 

fArch.  f.  Entwickelungsmechanik,  Bd,  viii.  p,  112, 


CHAPTER  II. 

DIMOEPHISM  AND   DISCONTINUOUS   VAKIATION. 

Dimorphism  in  the  earwig  and  in  the  crab — How  to  distinguish  be- 
tween species  and  varieties,  as  instanced  by  dimorphism  in 
certain  fishes,  and  in  a  marsh  plant — Polymorphism  in  plants — 
Series  of  Fibonacci — Discontinuous  variation  in  animals  as  regards 
vertebrae,  ribs,  mammae,  teeth,  digits,  and  other  characters — 
Homoeosis — De  Vries'  Theory  of  Mutation— Dimorphism  may  be 
due  to  internal  causes,  or  the  result  of  divergent  evolution — 
Physiological  Selection— Infertility  between  varieties. 

WE  have  seen  that  the  distribution  of  variations  about 
their  mean  is  in  many  cases  quite  symmetrical,  whilst 
in  other  cases  in  which  it  is  asymmetrical  it  still  takes 
place  according  to  some  orderly  arrangement,  for  which 
a  mathematical  expression  can  be  found.  There  is  still 
a  third  group  of  cases,  however,  in  which  the  curve  of 
distribution  is,  as  a  rule,  very  asymmetrical,  but  for 
which,  even  if  symmetrical,  no  single  general  mathe- 
matical expression  is  obtainable.  A  study  of  such 
curves  has  taught  us  that  the  cause  is  frequently  refer- 
able to  the  fact  that  our  material  is  not  homogeneous; 
that,  in  fact,  we  have  a  mixture  of  varying  numbers  of 
two  or  more  groups  of  individuals  differing  in  mean 
size  and  range  of  variation.  For  instance,  Bateson  * 
measured  the  length  of  the  forceps  of  583  specimens  of 
the  common  earwig,  Forficula  auricularia,  which  had 

*  ••  Materials  for  the  Study  of  Variation,"  p.  41. 
37 


38 


DISCONTINUOUS  VARIATION. 


been  collected  at  random  in  one  day  in  the  Fame 
Islands.  Only  mature  males  with  elytra  fully  devel- 
oped were  measured.  The  range  of  variation  was  from 
2.5  to  9.0  mm.,  the  various  lengths  occurring  with  a 
frequency  indicated  by  the  accompanying  curve.  Here 


120 


100 


60 


40 


20 


\ 


4567 
.Length  ofi  Forceps  in  <mm. 


8 


FIG.  7.— Distribution  of  various  lengths  of  forceps  in  the 
male  earwig. 

we  see  that  the  commonest  kinds  had  a  length  of  either 
3.5  mm.,  or  7  mm.,  or  were  of  the  forms  shown  in  Fig. 
8;  whilst  individuals  of  intermediate  length  occurred 
comparatively  infrequently.  This  species  was  there- 
fore most  distinctly  dimorphic  in  respect  of  the  char- 
acter measured,  and  it  maintained  this  dimorphism  in 
spite  of  the  fact  that  the  varying  individuals  were  liv- 


DISCONTINUOUS  VARIATION. 


39 


ing  in  close  communion  with  each  other  under  the  same 
stones. 

In  this  instance  there  could  be  no  doubt  as  to  the  di- 
morphism, just  as  in  many  of  the  instances  previously 
quoted  there  could  be  practically  no  doubt  as  to  the 
monomorphism,  but  obviously  there  must  be  inter- 
mediate stages  in  which  the  fusion  is  much  closer.  In 
these  an  irregular  asymmetrical  curve  with  only  one 
summit  may  be  obtained,  instead  of  a  distinct  double 


FIG.  8. — I.  High  male,  II.  Low  male,  of  Common  Earwig. 

humped  curve.  Weldon  *  obtained  such  a  curve  for 
the  distribution  of  the  frontal  breadths  of  Naples  speci- 
mens of  the  crab,  Carcinus  mcenas.  This  is  repro- 
duced in  Fig.  9,  the  horizontal  scale  representing  thou- 
sandths of  the  carapace  length,  and  the  vertical  scale 
numbers  of  individuals.  It  seemed  to  Professor  Wel- 
don very  probable  that  this  asymmetrical  curve  was 
produced  by  the  fusion  of  two  races  of  individuals,  clus- 
tered symmetrically  about  separate  mean  magnitudes, 
and  Professor  Pearson  tested  this  supposition  for  him. 
Pearson  calculated  that  by  mixing  41.45  per  cent,  of 
individuals  with  a  mean  frontal  breadth  of  630.62  thou- 

*Proc.  Roy.  Soc.,  liv.  p.  324,  1893. 


40 


DISCONTINUOUS  VARIATION. 


sandths,  and  a  probable  error  of  12.06,  with  58.55  per 
cent,  of  individuals  of  mean  breadth  654.66  and  prob- 
able error  8.41  (i.  e.y  groups  of  individuals  represented 
by  the  two  lower  dotted  line  curves  in  the  Figure),  the 
upper  dotted  line  curve  would  be  obtained.  It  will  be 
seen  that  this  corresponds  very  closely  with  the  ob- 


&>- 


-30- 


580      5UO    600     610     620     630      (JM     650.     660      t>70     60      6.90     700 

FIG.  9.— Distribution  of  frontal  breadths  of  Carcinua  mcenas. 

served  values,  and  so  supports  Weldon's  hypothesis.  It 
is  somewhat  curious  that  of  all  the  22  series  of  measure- 
ments made  by  Weldon  on  Naples  and  Plymouth  crabs, 
this  was  the  only  characteristic  in  respect  of  which 
dimorphism  was  exhibited.  As  an  explanation  of  it, 
Giard  *  has  suggested  that  one  of  the  two  groups  owed 
its  altered  frontal  breadth  to  the  presence  of  an  internal 
parasite,  Portunion  mcenadis.  Thus  he  measured  five 
specimens  of  C.  mcenas  infested  by  this  parasite,  and 
found  that  their  mean  relative  frontal  breadth  very 
*  Comptes  Rendus,  cxviii.  p.  870,  1894. 


DISCONTINUOUS  VARIATION.  41 

nearly  corresponded  to  the  lower  mean  value  of  Wei- 
don's  crabs  (viz.,  630.32  as  against  630.62).  Giard 
thinks  that  the  dimorphism  in  the  length  of  the  forceps 
of  the  earwig  observed  by  Bateson  can  be  similarly  ex- 
plained, for  the  short  individuals  appear  to  be  infested 
with  Gregarines,  and  the  longer  ones  not.  He  does 
not  wish  to  insist,  however,  that  all  dimorphism  is  the 
result  of  parasitic  influence,  but  merely  that  it  may  be 
so  in  certain  instances.  It  is  obvious,  indeed,  that  be- 
tween two  absolutely  distinct  varieties  or  species,  and 
between  pure  monomorphic  forms,  all  intermediate 
stages  may  exist.  But  how  are  these  intermediate 
stages  to  be  classified?  When  is  one  justified  in  assum- 
ing the  existence  of  two  distinct  species,  and  when  of 
only  one  species  with  an  increased  range  of  variation 
and  perhaps  a  tendency  to  split  up?  To  overcome  this 
difficulty  Davenport  and  Blankinship  *  have  suggested 
that  in  order  to  decide  in  any  given  case  whether  we  are 
dealing  with  two  or  more  confluent  species,  or  only  with 
varieties,  the  following  procedure  should  be  adopted: 
First  of  all  one  should  determine  the  most  distinctive 
character  of  the  members  of  the  group,  and  after  mak- 
ing a  series  of  measurements  in  respect  of  this  char- 
acter, plot  out  a  curve  showing  the  relative  frequency 
of  occurrence  of  each  measurement.  Supposing  that 
in  this  way  a  double  humped  curve  is  obtained,  then 
the  degree,  of  isolation  of  the  constituent  races  is  esti- 
mated by  measuring  the  depth  of  the  depression  be- 
tween the  two  humps,  from  the  level  of  the  maximum 
of  the  lower  hump.  This  depth  may  be  expressed  as  a 
percentage  on  the  length  of  the  maximum  ordinate,  or 
*  Science,  N.  S.  vol.  vii.  p.  685, 


42 


DISCONTINUOUS  VAEIATION. 


mode,  of  the  lower  hump.  The  value  so  obtained  is 
termed  by  Davenport  and  Blankinship  the  "  Index  of 
Isolation."  They  suggest  that  if  this  index  be  over  50 
per  cent.,  then  one  should  agree  to  look  upon  the  two 
groups  as  distinct  species;  if  under  50  per  cent.,  then 
only  as  varieties.  As  an  example,  they  adduce  a  case 
of  two  doubtful  species  of  fishes,  Leuciscus  balteatus, 
and  L.  hydrophlox,  which  differ  in  the  number  of  rays 
in  the  anal  fin.  On  plotting  out  the  frequencies  of  oc- 


w 
50 
40 
30 
20 
10 
0 

I 

\ 

1 

\ 

I 

\ 

/ 

'\ 

/ 

\ 

^ 

/ 

\ 

*-~~^ 

->^ 

/ 

/ 

^> 

^- 

12         14          16          18          20         22 

FIG.  10. — Distribution  of  fin  rays  in  Leuciscus. 

currence  of  the  various  numbers  of  rays  in  194  indi- 
viduals, the  curve  given  in  Fig.  10  was  obtained.  In 
this  case  the  index  of  isolation  is  exactly  50  per  cent., 
hence  we  are  just  at  the  limit  of  species  and  varieties. 
The  importance  of  determining  which  is  the  most 
distinctive  character,  before  drawing  any  conclusions 
from  the  indices  of  isolation  found,  is  well  shown  by  an- 
other case  adduced  by  these  authors.  It  concerns  the 
marsh  plant  Typha,  which  is  found  in  the  eastern 
United  States.  Seven  characters  regarded  as  probably 
specific  were  measured  in  about  250  specimens,  which 
had  been  collected  at  distances  of  about  one  metre 
apart  across  the  swamps  in  which  it  occurred.  The 
variation  curves  obtained  in  the  case  of  the  stem  height, 


DISCONTINUOUS  VARIATION. 


43 


diameter  of  stem  taken  at  half  the  height,  and  the  width 
of  the  largest  leaf  at  its  widest  part,  are  reproduced  in 
Figs.  11,  12,  and  13.  Here  we  see  that  the  stem 
height  shows  no  differentiation,  the  curve  being  more 
or  less  symmetrical.  The  mid-stem  diameter  shows  a 
slight  second  hump,  but  this  is  obviously  insufficient  to 
indicate  the  presence  of  two  species.  The  leaf  width, 


40- 


20- 


10- 


NO.:  7 

Indv. 


r 


Dm.t    8   9   1011121314151617181920  21 

Stem-Height 
FIG.  11.— Distribution  of  Stem-Heights  in  Typha. 


however,  shows  a  marked  differentiation,  the  index  of 
isolation  being  75  per  cent.  Of  the  other  characters 
measured,  the  diameter  of  the  stem  at  its  base,  the 
diameter  of  the  pistillate  spike,  and  the  interval  be- 
tween the  staminate  and  pistillate  spikes,  had  indices  of 
isolation  of  respectively  79,  89,  and  83  per  cent.,  or 
showed  even  greater  differentiation  than  the  leaf  width. 
The  curve  for  the  pistillate  spike  length  was,  however, 
symmetrical.  Thus  this  plant  shows  distinct  differen- 


44 


DISCONTINUOUS  VARIATION. 


tiation  into  two  species  in  respect  of  four  out  of  the 
seven  characters  measured,  and  so  is  obviously,  in  the 
authors'  opinion,  to  be  regarded  as  composed  of  two 
more  or  less  confluent  species. 

This  "  precise  criterion  of  species "  suggested  by 
Davenport  and  Blankinship  has  much  to  recommend  it, 
but  probably  it  would  generally  be  considered  that  an 
index  of  isolation  of  only  50  per  cent,  is  too  small  a  dif- 
ference to  merit  specific  distinction.  Perhaps  it  would 


70 


40 


20 


4       6       8      10     12mm. 
Mid-Stem  Diameter 


FIG.  12.— Distribution  of  Mid-Stem  Diameters  in  TypTia. 

be  better,  therefore,  to  increase  it  to  90  or  95  per  cent. 
In  any  case,  it  must,  I  think,  be  admitted  that  a  slight 
degree  of  confluency,  or  overlapping  of  the  curves  of 
variation,  ought  not  to  compel  one  to  assume  the  exist- 
ence of  only  a  single  species,  though  this  is  the  view 
which  has  been  generally  held  in  the  past.  On  the 
other  hand,  the  fact  of  a  group  of  organisms  showing 
absolutely  discontinuous  variation  in  respect  of  some 
apparently  unimportant  characteristic  ought  not  to 


DISCONTINUOUS  VAKIATION. 


45 


compel  one  to  regard  it  as  composed  of  two  distinct 
species.  It  is,  of  course,  often  impossible  to  tell  whether 
any  given  characteristic  is  important  or  not,  and  hence 
we  must  recognise  that  a  really  precise  and  univer- 


FIG.  13.— Distribution  of  Leaf- Widths  in  Typha. 

sally  applicable  definition  of  a  species  is,  and  always 
must  be,  unattainable. 

It  is  probable  that  variation  series  in  the  Vegetable 
Kingdom  often  give  double  humped  or  multiple 
humped  curves,  even  if  the  material  examined  is  as 
homogeneous  as  it  is  possible  to  obtain  it.  Possibly,  if 
only  individuals  of  the  same  stock  were  examined, 
they  would  be  found  to  give  single  humped  curves,  but 
if  material  collected  from  different  parts  of  the  same 
district,  or  even  of  the  same  field,  is  to  be  regarded  as 
composed  of  so  many  local  races  or  sub-varieties,  then 
the  determination  of  the  variations  in  many  plant 


46  DISCONTINUOUS  VARIATION. 

species  would  become  an  almost  hopeless  task.  The 
degree  to  which  local  races  may  vary  is  well  shown  by 
some  of  Ludwig's  determinations.  For  instance,*  four 
groups  of  specimens  of  Torilis  anthriscus,  obtained 
from  various  spots  near  Schmalkalden,  had  the  follow- 
ing numbers  of  branches  of  the  main  umbels : 

34         5        67       89       10    11    12    13    Total 


Group 

I. 

1 

11 

18 

45 

28 

20 

5 

4 

1 

133 

" 

II. 

1 

5 

8 

13 

25 

12 

5 

2 

71 

«i 

III. 

5 

7 

9 

8 

12 

8 

1 

1 

51 

" 

IV. 

7  60  213 

152 

46 

18 

3 

1 

500 

In  the  first  group,  the  8  branch  form  occurred  the 
most  frequently,  the  variations  from  this  number  being 
distributed  more  or  less  evenly  around  it.  In  the  next 
group,  the  10  branch  form  was  the  commonest;  in  tlie 
third  group  the  8  and  10  branch  forms  were  both  com- 
mon, whilst  in  the  last  group  of  all,  collected  in  a  wood 
at  Wolfsberg,  near  to  Schmalkalden,  a  quite  distinct 
race  having  five  branches  presented  itself. 

In  determining  the  variations  of  a  plant,  therefore, 
it  is  probably  best  to  obtain  a  very  large  amount  of  ma- 
terial, from  various  sources,  and  submit  this  to  examina- 
tion. Though  the  curve  thereby  obtained  may  be  very 
composite,  yet  at  least  it  will  indicate  something  as 
to  the  range  of  variation  of  the  flower  in  many  of  its 
local  races,  and  also  what  number  of  branches  or  other 
parts  occur  on  the  whole  most  frequently.  For  in- 
stance, Ludwig  f  has  had  made  enumerations  of  the 
number  of  ray  florets  in  17,000  specimens  of  the  Ox-eye 
Daisy,  Chrysanthemum  leucanthemum.  The  material 

*Bot.  Centralb.,  vol.  Ixiv.  p.  40. 
fBot.  Centralb.,  vol.  Ixiv.  p.  1,  1895. 


DISCONTINUOUS  VAKIATION. 


47 


was  collected  from  various  sources,  between  the  years 
1890-95,  and  was  examined  by  various  people.  The 
number  of  florets  varied  from  7  to  43,  the  following 
being  the  frequency  of  their  occurrence: 


« 

N 

o 

. 

kj 

g 

• 

g 

1 

i 

i 

| 

§ 

| 

• 
• 

g 

1 

1 

P 

I 

1 

§ 

fc 

D 

| 

I 

1 

m 

& 

£ 

£ 

7 

2 

16 

479 

25 

602 

34 

346 

8 

9 

17 

525 

26 

614 

35 

186 

9 

13 

18 

625 

27 

375 

36 

64 

10 

36 

19 

856 

28 

377 

37 

28 

11 

65 

20 

1568 

29 

294 

38 

16 

12 

148 

21 

3650 

30 

196 

39 

16 

13 

427 

22 

1790 

31 

183 

40 

14 

14 

383 

23 

1147 

32 

187 

41 

0 

15 

455 

24 

812 

33 

307 

42 

3 

43 

2 

Those  results  are  plotted  out  in  the  form  of  a  curve 
in  Fig.  14.  Here  we  see  that  the  21  floret  form  occurs 
by  far  the  most  frequently,  but  that  there  are  also 
secondary  smaller  maxima  or  humps  on  the  curve  for  13, 
26,  and  34  floret  forms.  This  curve  thus  gives  one  a 
good  idea  both  as  to  the  range  of  variation  of  the  num- 
ber of  florets  in  this  flower,  and  also  as  to  the  most  fre- 
quently occurring  forms.  Enumerations  of  small  num- 
bers of  specimens  of  local  races  showed  the  13  floret 
form  to  be  the  commonest  form  in  one  case,  and  the  34 
floret  form  in  another,  but  these  results  obviously  fail  to 
give  a  true  idea  of  the  variation  of  the  plant. 

Similar  enumerations  of  the  florets  of  various  other 
species  of  the  Composite?  showed  the  following  to  be 
the  most  frequently  occurring  numbers  of  ray  florets, 


48  DISCONTINUOUS  VAEIATION. 

the  absolute  maximum  being  in  each  case  indicated  by 
thick  type :  * 

Chrysanthemum  leucanthemum,  13       21        34       26 

"  inodorum,  13       21 

"  segetum        .  13        21 


Anthemis  arvensis, 

Cotula, 

"        tinctoria, 
Achillea  ptarmica, 
Senecio  nemorensis, 
Fuchsii, 


5     8       13 
8     13 

21 

8       13  10 

3        5 
3       5 


Taking  the  observations  as  a  whole,  we  see  that  the  most 
frequently  occurring  numbers  are  the  following: 

3        5        8        10        13        21        26        34 
In  various  species  of  the  Umbellifercz  the  following  are 
the  most  frequently  occurring  numbers  of  petals: 

3  5  (10  15  20  25)  8  13  21  34 
That  is  to  say,  in  each  case  the  numbers  follow  the  so- 
called  series  of  Fibonacci,  viz.,  1,  2,  3,  5,  8,  13,  21,  34, 
etc.,  in  which  each  number  is  the  sum  of  the  two  before 
it;  or  else  they  follow  some  multiples  of  these  numbers. 
Ludwig  states  that  this  relationship  is  by  no  means 
limited  to  the  two  orders  of  plants  mentioned,  but  that 
it  extends  to  other  members  of  the  Vegetable  Kingdom, 
and  probably  also  to  the  Animal  Kingdom. 

Perhaps  the  most  striking  result  that  Ludwig  t  ob- 
tained was  for  the  number  of  petals  of  one  of  the  Prim- 
roses, Primula  officinalis.  This  varied  from  1  to  22  in 
the  sample  of  1170  flowers  examined.  These  flowers 
were  all  obtained  from  a  single  meadow  (near  Wieda), 
and  were  therefore  as  homogeneous  as  it  was  possible  to 

*Bot.  Centralb.,  vol.  Ixiv.  p.  100. 

|Ber.  d.  deutsch.  bot.  Gesell.,  xiv.  p.  204,  1896. 


DISCONTINUOUS  VARIATION. 


49 


obtain  them.  The  frequency  of  occurrence  of  the  vari- 
ous forms  is  indicated  in  Fig.  15,  given  below. 
Here  it  will  be  seen  that  the  curve  has  five  most  dis- 


8500 


85       40 


Number  of  Ray=-f  lorets. 
FIG.  14. — Distribution  of  Ray-florets  in  the  Ox-eye  daisy. 

tinct  maxima,  corresponding  to  3,  5,  8,  10,  and  13  petal 
forms. 

That  these  multi-humped  curves  are  actually  due  to 
a  mingling  of  two  or  more  local  races  is  proved  by  an 


50 


DISCONTINUOUS  VARIATION. 


interesting  experiment  of  De  Vries.*  He  sowed  the 
mixed  seed  of  Chrysanthemum  segetum  obtained  from 
twenty  different  gardens.  The  topmost  flowers  of  the 


024          6         8         10       12         14         16        18        20 
Kumbei:  of  petals 

PIG.  15.— Distribution  of  Petals  in  Primula  officinalis. 


chief  stem  of  each  of  the  97  healthy  plants  obtained 
were  examined,  and  were  found  to  contain  the  follow- 
ing numbers  of  ray  florets: 

Ray  florets,     12     13      14      15     16      17      18      19 
Frequencies,     114      13        4       6        9        7      10 

*Arch.  f.  Entwickelungsmechanik,  Bd.  ii.  p.  52,  1896. 


21 
20 


DISCONTINUOUS  VARIATION.  51 

Thus  there  were  obviously  two  forms  present,  a  13  ray 
form  and  a  21  ray  form.  The  seeds  from  the  12  and 
13  ray  forms  were  collected  and  sown  next  year,  the 
flowers  obtained  therefrom  having  the  following  num- 
bers of  florets: 

Ray  florets,  8  9  10  11  12  13  14  15  16  17  18  19  20  21 
Frequencies,  2  107  13  94  25  7712030 

That  is  to  say,  all  trace  of  the  21  ray  form  had  been 
eliminated,  and  a  nearly  pure  13  ray  form  obtained. 
That  this  was  so  was  proved  by  sowing  the  seed  of  some 
of  the  12  rayed  plants  obtained  on  this  occasion  in  the 
following  year.  It  was  then  found  that  the  frequencies 
of  occurrence  of  flowers  with  various  numbers  of  rays 
remained  practically  unchanged. 

But  how  do  these  cases  of  what  Bateson  has  termed 
discontinuous  variation  arise  ?  In  one  or  two  of  the  in- 
stances quoted  we  saw  that  the  two  humps  of  the  curve 
of  variation  scarcely  overlapped  at  all.  In  the  case  of 
Primula  they  were  all  of  them  sharply  defined,  but 
there  was  still  a  good  deal  of  fusion,  whilst  in  the  ray 
florets  of  the  Ox-eye  Daisy  the  fusion  was  greater  still. 
Finally,  in  the  frontal  breadths  of  Naples  crabs  the 
fusion  was  complete,  and  the  existence  of  dimorphism 
was  shown  only  by  the  asymmetry  of  the  curve.  It 
would  be  possible  to  multiply  instances  of  such  curves 
as  these,  in  which  the  fusion  ran  through  all  stages  of 
completeness  and  incompleteness,  but  those  quoted  are 
quite  sufficient  for  our  purpose.  They  suffice  to  show 
that  all  stages  of  fusion  may  be  met  with,  and  so  incline 
one  to  the  opinion  that  the  later  stages,  in  which  the  two 
or  more  humps  of  the  curve  overlap  little  if  at  all,  are 


52  DISCONTINUOUS  VARIATION. 

but  more  advanced  stages  of  those  curves  in  which  the 
fusion  is  nearly  complete.  In  fact  they  seem  to  indi- 
cate that  if  only  the  ancestry  of  such  varying  organisms 
could  be  traced  backwards  continuously,  it  would  be 
found  that  at  no  period  was  there  any  sudden  change 
from  continuity  to  discontinuity;  that  a  condition  of 
absolute  dimorphism,  or  formation  of  two  new  species, 
was  merely  evolved  by  very  gradual  and  almost  imper- 
ceptible steps  from  the  original  pure  monomorphism. 
This  is,  I  believe,  the  opinion  held  by  the  majority  of 
naturalists  at  the  present  day  as  to  the  origin  of  by  far 
the  larger  number  of  cases  of  dimorphism,  but  dissen- 
tient voices  have  not  been  entirely  wanting.  Thus  Gal- 
ton  *  is  of  the  opinion  that  the  aberrant  or  discontinu- 
ous variations  generally  known  as  sports  may  be  of  con- 
siderable significance  in  evolution.  Because  evolution 
may  proceed  by  minute  steps,  he  considers  that  it  does 
not  by  any  means  follow  that  it  must  so  proceed.  Again, 
within  recent  years  the  orthodox  view  has  been  ably 
combated  by  Bateson  f  in  his  book  on  Variation.  In 
this  work  he  has  collected  a  very  large  number  of  in- 
stances of  discontinuous  variation,  or  variations  in  re- 
spect of  certain  organs  or  parts,  which  have  suddenly 
arisen  in  a  complete  and  perfect  state,  without,  as  a 
rule,  the  occurrence  of  any  intermediate  stages.  If, 
therefore,  argues  Bateson,  such  instances  of  discontinu- 
ous variation  undoubtedly  occur,  is  it  not  possible  that 
the  Discontinuity  of  Species  which  is  so  striking  a  fact 
amongst  living  organisms  is  a  consequence  and  expres- 
sion of  this  discontinuity  of  variation?  Thus  the  view 

*  "  Natural  Inheritance,"  p.  32,  1889. 

f  "  Materials  for  the  Study  of  Variation,"  London,  1894. 


DISCONTINUOUS  VARIATION.  53 

hitherto  generally  held,  since  Darwin  first  gave  ex- 
pression to  it,  is  that  almost  all  variations  are  very 
slight,  and  form  a  continuous  series.  It  is  only  by  their 
very  slow  accumulation,  therefore,  under  the  action  of 
Natural  Selection  and  other  agencies,  that  species  as  we 
know  them  have  been  evolved.  This  view  of  Bateson's 
is  so  striking  and  important  that  it  behoves  us  to  ex- 
amine it  at  some  little  length.  It  will  enable  us  to  ob- 
tain a  clearer  idea  of  Bateson's  views  if  we  indicate  his 
system  of  classification.  Thus  he  points  out  that  varia- 
tions are  divisible  into  two  classes,  substantive  and 
meristic.  Substantive  variations  are  variations  occur- 
ring in  the  actual  constitution  or  substance  of  the  parts 
themselves.  Meristic  variations,  on  the  other  hand,  are 
those  which  relate  to  the  number  of  parts  in  organisms. 
For  instance,  the  flower  of  the  Narcissus  is  commonly 
divided  into  six  parts,  but  through  meristic  variation  it 
may  be  divided  into  seven  parts,  or  only  four.  Never- 
theless, there  is  in  such  a  case  no  perceptible  change  in 
the  tissue  or  substance  of  which  the  parts  are  made  up. 
On  the  other  hand  many  Narcissi,  N.  corbularia,  for 
example,  are  known  in  two  colours,  one  a  dark  yel- 
low, and  the  other  a  sulphur  yellow,  though  the 
number  of  parts  and  pattern  of  the  flowers  are  identi- 
cal. This  is,  therefore,  an  example  of  a  substantive 
variation. 

Bateson  considers  that  there  can  be  no  doubt  that 
these  two  classes  of  variation  are  essentially  distinct 
from  each  other.  It  is  obvious  that  all  cases  of  meris- 
tic variation  are  also  cases  of  discontinuous  variation, 
whilst  cases  of  substantive  variation  are  much  more  fre- 
quently continuous  than  discontinuous.  It  is  to  be  noted 


54  DISCONTINUOUS  VAEIATION. 

that  these  discontinuous  meristic  variations  are  not  only 
large,  but  they  are  complete  and  perfect.  But  after 
all  one  would  scarcely  expect  anything  else.  Between  a 
six  petal  and  a  seven  petal  flower  it  is  scarcely  pos- 
sible to  imagine  such  a  thing  as  a  really  intermediate 
stage.  Even  if  one  found  a  flower  with  six  normal 
petals  and  a  seventh  abnormally  small  one,  or  five  nor- 
mal ones  and  a  sixth  in  process  of  dividing  into  two, 
one  would  scarcely  be  justified  in  regarding  it  as  an  in- 
termediate form,  for  the  flower  would  no  longer  be  sym- 
metrical. 

Perhaps  the  most  interesting  part  of  Bateson's  work 
lies  in  the  cases  which  he  has  collected  of  what  he  terms 
Homceosis.  By  this  he  means  those  variations  which 
consist  in  the  assumption  by  one  member  of  a  meristic 
series,  of  the  form  and  characters  proper  to  other  mem- 
bers of  the  series.  For  instance,  Kraatz  has  described 
a  saw-fly,  Cimbex  axillaris,  having  the  peripheral  parts 
of  the  left  antenna  developed  as  a  foot,  the  right  an- 
tenna being  normal.  Kriechbaumer  has  described  a 
nearly  similar  condition  in  a  Humble-bee,  Bombus 
variabilis.  Bateson  has  himself  described  a  crab, 
Cancer  pagurus,  having  the  right  third  maxillipede  de- 
veloped as  a  chela.  Milne-Edwards  has  described  an- 
other crab,  Palinurus  penicillatus,  in  which  the  left 
eye  bore  an  antenna-like  flagellum  several  centimetres 
in  length,  growing  up  from  the  surface  of  the  eye.  The 
eye  stalk  appears  to  have  been  of  normal  shape,  but  re- 
duced in  size.  Other  instances  somewhat  similar  to 
these  are  adduced,  but  it  is  unnecessary  to  quote  them 
here.  It  may  be  mentioned,  however,  that  most  inter- 
esting examples  of  this  form  of  variation  have  been 


DISCONTINUOUS  VARIATION.  55 

recently  obtained  by  Herbst.*  He  found  in  no  less 
than  ten  different  species  of  Crustacea,  belonging  to 
four  different  families,  that  if  an  eye  stalk  were  totally 
extirpated,  there  always  grew  up  in  its  place  a  hetero- 
morphic  new  structure,  like  an  antennula,  which  bore 
olfactory  hairs.  If,  however,  only  the  eye  were  re- 
moved, and  the  stalk  together  with  the  ganglion  left, 
instead  of  the  antennula  there  arose  the  beginnings  of 
a  new  eye.  A  similar  result  to  this  was  obtained  on  ex- 
tirpation of  either  stalk  or  eye  in  Porcellana  platycheles, 
presumably  because  in  this  species  the  stalk  contains  no 
ganglion. 

The  chief  contents  of  Bateson's  book  may  be  very 
briefly  summarised,  in  order  that  the  reader  may  gather 
some  idea  as  to  the  kind  of  evidence  on  which  Bateson 
founds  his  argument.  A  considerable  body  of  evidence 
is  given  concerning  variations  in  the  numbers  of  ver- 
tebrae and  ribs,  the  most  important  conclusions  being 
that  the  variations  are  considerable,  especially  in  some 
types  such  as  Simia  satyrus,  the  Bradypodidse,  and 
Bombinator  igneus,  and  that  imperfect  vertebrae  are 
very  rare.  Turning  to  Invertebrates,  it  is  shown  that 
among  Oligochaeta  and  Hirudinea,  certain  forms,  e.  g., 
Perionyx  excavatus  and  PacJiydrilus  sphagnetorum, 
have  great  variability,  whilst  others,  such  as  the  com- 
mon earthworm,  rarely  vary.  Both  forward  and  back- 
ward Homoeosis  may  occur,  forms  which  normally  have 
the  male  pores  on  the  15th  segment  having  them  on  the 
16th,  or  on  the  13th.  Returning  to  Vertebrates,  evi- 
dence is  next  adduced  concerning  cervical  fistulae  and 
auricles,  and  supernumerary  mammae.  Variations  in 
*  Arch.  f.  Entwickelungsmechanik,  Bd.  ix.  p.  215,  1899. 


56  DISCONTINUOUS  VARIATION. 

teeth  are  dealt  with  in  very  great  detail,  and  conclu- 
sions are  drawn  as  to  the  comparative  frequency  of 
dental  variation  in  various  animals.  The  animals  show- 
ing the  greatest  frequency  of  extra  teeth  are- domestic 
dogs,  Anthropoid  apes,  and  the  Phocidse.  It  is  espe- 
cially noticeable  that  the  variability  of  domestic  animals 
in  respect  of  teeth  is  not  markedly  in  excess  of  that  seen 
in  wild  forms.  Thus,  though  supernumerary  teeth  are 
more  common  in  domestic  dogs  and  cats  than  in  wild 
Canidse  and  Eelidse,  they  are  not  more  so  than  in  An- 
thropoid apes  and  in  the  Phocidse.  With  respect  to  the 
question  of  symmetry,  the  evidence  shows  that  dental 
variation  may  be  symmetrical  on  the  two  sides,  but  that 
much  more  frequently  it  is  not  so.  Other  evidence  is 
given  concerning  the  division  of  teeth,  the  presence  and 
absence  of  first  premolars  and  last  molars,  the  least  size 
of  particular  teeth,  and  other  subjects. 

Variations  in  the  number  of  digits  are  treated  more 
fully  than  any  other  subject  discussed,  though  the  evi- 
dence adduced  is  stated  to  bear  rather  on  morphological 
conceptions  than  in  any  direct  manner  on  the  problem 
of  Species.  It  is  found  that  the  frequency  of  digital 
variation  is  immensely  greater  in  some  classes  of  species 
than  in  others.  Thus  the  horse  shows  many  recorded 
cases,  but  the  ass  none  at  all.  Variation  is  common  in 
the  cat,  pig,  fowl,  and  pheasant,  but  rare  in  the  dog, 
sheep,  and  in  most  birds.  In  the  cat,  ox,  horse,  pig,  and 
in  man,  the  digital  variation  approaches  to  particular 
forms,  and  has  in  it  something  distinctive.  Digital 
variation  is  sometimes  symmetrical,  but  more  often 
asymmetrical. 

In  other  chapters  of  the  book  is  found  a  considerable 


DISCONTINUOUS  VARIATION.  57 

mass  of  data  concerning  the  repetition  and  division  of 
appendages  in  insects  and  Crustacea,  colour  markings 
and  colour  variations  in  Lepidoptera,  variations  in  the 
number  of  legs  of  different  species  of  Peripatus^  the 
occurrence  of  double  monsters,  and  various  other  sub- 
jects, but  to  these  it  is  unnecessary  to  refer  here. 
Sufficient  have  been  mentioned  to  indicate  the  general 
nature  and  scope  of  the  evidence,  so  that  we  are  enabled 
to  enquire  how  far,  if  at  all,  it  can  warrant  Bateson's 
hypothesis  as  to  the  origin  and  production  of  discon- 
tinuity in  species.  We  see  that  most  of  the  evidence  con- 
cerns obvious  abnormalities,  generally  in  the  direction 
of  increase  in  the  number  of  parts,  which  have  arisen 
suddenly  and  apparently  spontaneously.  In  practically 
no  case  has  any  new  structure  arisen,  but  only  a  repe- 
tition or  misplacement  of  those  already  present,  and  so 
it  is  difficult  to  understand  how  really  new  structures 
and  organs  could  have  originated,  even  if  it  be  admitted 
that  such  abnormalities  are  of  very  frequent  occur- 
rence, and  that  they  could  succeed  in  permanently 
establishing  themselves.  But  first  of  all  it  is  neces- 
sary to  point  out  that  the  large  majority  of  these  abnor- 
malities are  extraordinarily  rare,  occurring  perhaps  not 
once  in  100,000  or  once  in  a  million  cases.  What 
chance  have  they,  then,  of  establishing  themselves  on  a 
permanent  footing?  Bateson  remarks,  "  An  error  more 
far-reaching  and  mischievous  is  the  doctrine  that  a  new 
variation  must  immediately  be  swamped,"  but  he  fails 
to  adduce  one  tittle  of  evidence  to  prove  that  it  is  an 
error  at  all.  This  is  simply  because  no  such  evidence 
exists.  It  is  true  that  some  animals  are  prepotent  over 
others  in  procreating  their  characteristics,  and  their 


58  DISCONTINUOUS  VARIATION. 

abnormalities  if  they  possess  them,  but  this  prepotency 
is  quite  limited  in  its  range.  Darwin  in  his  "  Variation 
of  Animals  and  Plants  under  Domestication  "  *  men- 
tions an  instance  of  transmission  of  supernumerary 
digits  through  five  generations,  whilst  in  other  cases 
they  have  reappeared  after  an  interval  of  even  three 
generations.  "  But,"  says  Darwin,  "  we  must  not  over- 
estimate the  force  of  inheritance.  Dr.  Struthers  as- 
serts that  cases  of  non-inheritance  and  of  the  first  ap- 
pearance of  additional  digits  in  unaffected  families  are 
much  more  frequent  than  cases  of  inheritance." 

Unless  much  stronger  evidence  than  that  hitherto  ad- 
vanced be  obtained,  it  therefore  follows  that,  according 
to  the  known  laws  of  inheritance,  suddenly  occurring 
variations,  unless  artificially  selected,  must  inevitably 
be  swamped  by  intercrossing  and  disappear.  Suppos- 
ing, on  the  other  hand,  any  such  variation  is  artificially 
isolated,  and  bred  in  and  in  with  its  own  offspring,  then 
it  may  be  possible  to  establish  a  distinct  race,  bearing  in 
undiminished  degree  all  the  abnormal  characteristics  of 
the  original  variety.  For  instance,  Darwin  thus 
records  the  origin  of  the  ancon  sheep :f  "In  1791  a 
ram-lamb  was  born  in  Massachusetts,  having  short 
crooked  legs  and  a  long  back,  like  a  turnspit  dog. 
From  this  one  lamb  the  otter  or  ancon  semi-monstrous 
breed  was  raised ;  as  these  sheep  could  not  leap  over  the 
fences,  it  was  thought  that  they  would  be  valuable. 
The  sheep  are  remarkable  for  transmitting  their  char- 
acter so  truly  that  Colonel  Humphreys  never  heard  of 
'  but  one  questionable  case  '  of  an  ancon  ram  and  ewe 

*Vol.  i.  p.  457,  Ed.  ii. 
\Ibid.,  vol.  i.  p.  104. 


DISCONTINUOUS  VARIATION.  59 

not  producing  ancon  offspring."  Again,  Darwin  says:  * 
"  It  is  certain  that  the  ancon  and  the  mauchamp  breeds 
of  sheep,  and  almost  certain  that  the  niata  cattle,  turn- 
spit and  pug-dogs,  jumper  and  frizzled  fowls,  short- 
faced  tumbler  pigeons,  hook-billed  ducks,  etc.,  suddenly 
appeared  in  nearly  the  same  state  as  we  now  see  them. 
So  it  has  been  with  many  cultivated  plants."  Ajid 
then  he  adds,  "  The  frequency  of  these  cases  is  likely  to 
lead  to  the  false  belief  that  natural  species  have  often 
originated  in  the  same  abrupt  manner.  But  we  have 
no  evidence  of  the  appearance,  or  at  least  of  the  con- 
tinued procreation,  under  nature,  of  abrupt  modifica- 
tions of  structure." 

We  see  therefore  that,  though  Darwin  brought  for- 
ward much  more  powerful  and  convincing  instances  of 
discontinuous  variation  than  those  cited  by  Bateson,  he 
held  them  to  be  quite  inadequate  to  account  in  any  way 
for  the  discontinuity  observed  in  species. 

Under  the  title  of  "  Die  Mutationstheorie"  De  Vries 
has  recently  promulgated  views  concerning  the  origin 
of  species  which  are  somewhat  similar  to  those  held  by 
Bateson.  The  evidence  he  adduces  in  support  of  them  is 
chiefly  derived  from  observations  of  his  own  on  flower- 
ing plants,  and  even  if  his  theoretical  views  be  entirely 
rejected,  there  can  be  no  doubt  as  to  the  intrinsic 
interest  and  importance  of  the  observations  themselves. 
According  to  the  theory  of  mutation,  the  qualities  of 
organisms  are  built  up  of  individual  units  sharply 
defined  from  each  other.  When,  in  the  course  of 
evolution,  one  species  arises  from  another,  it  follows 
that  the  change  takes  place  by  a  distinct  step  or  jump, 
*Ittd.,  vol.  ii.  p.  409. 


60  DISCONTINUOUS  VARIATION. 

i.  e.,  is  discontinuous,  and  does  not  occur  gradually. 
Such  mutations  may  take  place  in  all  directions,  but 
probably  they  only  occur  from  time  to  time,  due,  per- 
haps, to  the  periodical  action  of  fixed  causes.  They 
are  distinct  from  the  slight  differences  observed  in 
local  races  and  varieties,  for  these  can  be  produced 
gradually  by  artificial  selection  and  changed  condi- 
tions of  environment.  Also  Natural  Selection  can 
only  lead  to  the  formation  of  such  local  races,  it  being 
powerless  to  bring  about  true  mutations.  The  varia- 
tion which  leads  to  the  formation  of  new  species,  there- 
fore, is  essentially  discontinuous,  not  continuous. 

In  order  to  obtain  evidence  in  support  of  his  theory, 
De  Yries  has  cultivated  over  100  different  species  of 
plants,  but  only  one  of  them,  (Enothera  Lamarckiana, 
showed  the  desired  mutations.  This  plant  was  origi- 
nally brought  to  Europe  from  America,  and  kept  under 
cultivation.  It  has  since  run  wild,  and  De  Vries  ob- 
tained the  stock  of  nine  plants  which  formed  his  first 
generation  from  a  field  near  Hilversum.  Unfortu- 
nately, the  true  origin  of  the  plant  is  obscure.  In 
Britton  and  Brown's  recently  issued  "  Flora  of  the 
United  States,"  no  reference  whatever  is  made  to  it  as 
a  wild  species.  Hence  it  is  probably  a  garden  variety  of 
(Enothera  biennis  (Evening  Primrose),  and  may  be  a 
hybrid  plant,  whilst  the  mutations  obtained  by  De 
Yries  may  merely  be  partial  or  complete  reversions  to 
the  original  ancestors  of  the  plant.  To  obtain  these 
mutations,  De  Yries  cultivated  the  plant  through  eight 
generations,  and  during  this  time,  obtained  over  50,000 
specimens.  Of  these,  834  showed  characters  which 
sharply  differentiated  them  from  the  normal  0.  La- 


DISCONTINUOUS  VARIATION.  61 

marckiana.  De  Yries  classified  them  as  follows:  350 
0.  oblonga;  229  0.  lata;  158  0.  nanella;  56  0.  albida; 
32  0.  rubrinervis;  8  0.  scintillans;  and  1  0.  gig  as.  Of 
these  new  "  species,"  oblonga,  albida,  rubrinervis, 
nanella,  and  gigas  remained  absolutely  constant  in  sub- 
sequent generations,  when  crossed  among  themselves, 
or  self -fertilised,  in  the  case  of  0.  gigas.  0.  scintillans 
was  not  nearly  so  constant,  the  offspring  yielding  only 
about  a  third  of  the  parent  form,  and  the  rest  of  them 
being  Lamarckiana,  oblonga •,  and  lata.  0.  albida  bred 
quite  constant,  but  the  plants  were  weak,  and  not  very 
fertile. 

De  Yries  looks  upon  his  sports  as  true  species,  and 
not  varieties,  for  he  says  that  varieties  differ  from  their 
parent  species  in  only  one  or  two  characters,  whilst 
species  differ  from  their  nearest  allies  in  almost  all  their 
characters.  Thus  in  comparison  with  the  parent  form, 
0.  Lamarckiana,  gigas  was  stronger  and  albida  was 
weaker,  both  forms  having  broader  and  shorter  leaves. 
The  flowers  of  gigas  were  larger,  those  of  rubrinervis 
darker  yellow,  those  of  oblonga  and  scintillans  smaller, 
and  those  of  albida  paler.  The  cuticle  of  albida  was 
rough.  The  bosses  on  the  leaves  of  lata  were  increased, 
and  on  those  of  scintillans  diminished.  The  forma- 
tion of  pollen  was  increased  in  rubrinervis  and  dimin- 
ished in  scintillans.  The  seeds  of  gigas  were  larger, 
and  those  of  scintillans  smaller;  those  of  rubrinervis 
more  abundant,  and  those  of  lata  more  scanty. 

By  artificial  selection  De  Yries  obtained  in  one  in- 
stance *  what  he  regards  as  a  true  mutation.  This  was 
in  the  case  of  Linaria  vulgaris  (yellow  toadflax). 
*  "  Die  Mutationstbeorie,"  p.  552, 


62  DISCONTINUOUS  VAEIATION. 

Starting  with  plants  which  had  one  or  two  peloric 
flowers,  he  bred  and  selected  them  through  several  gen- 
erations, and  ultimately  obtained  some  entirely  peloric 
plants.  These  plants  were  most  of  them  sterile,  but  a 
few  yielded  seeds,  and  ultimately  De  Yries  obtained  a 
peloric  race,  Linaria  vulgaris  peloria,  only  10  per  cent, 
of  the  seeds  of  which  reverted  to  hemipeloric  plants 
(i.  e.,  plants  with  some  peloric  and  some  non-peloric 
flowers).  According  to  De  Yries,  this  new  race  is  a 
true  mutation,  because  the  pure  peloric  plants  from 
which  it  was  derived  arose  suddenly,  and  apparently 
capriciously,  from  hemipeloric  parents.  He  also  ob- 
tained new  races  of  other  plants  by  artificial  selection 
extending  through  several  generations,  but  these  he 
regards  only  as  varieties,  and  not  true  species,  in  that 
their  formation  was  gradual.  However,  there  seems 
to  be  no  valid  ground  for  sharply  differentiating  them 
in  this  manner.  For  instance,  in  the  case  of  the  five- 
leaved  clover  race  (Tri folium  pratense  quinque folium) 
obtained  by  him,*  he  started  breeding  with  two  natu- 
rally occurring  clover  plants  which  had  four  leaflets  to 
their  leaves,  and  in  the  case  of  one  leaf,  five  leaflets. 
It  is  difficult  to  understand  why  these  naturally  occur- 
ring plants  should  not  be  regarded  as  true  mutations, 
just  as  much  as  the  peloric  race  above  mentioned,  or 
why,  indeed,  the  naturally  occurring  hemipeloric  plants 
from  which  the  peloric  race  was  obtained  were  not  like- 
wise true  mutations. 

In  the  case  of  the  clover,  the  breeding  was  continued 
through  several  generations,  the  seed  of  only  the  few 
plants  richest  in  four  or  more  leaflets  being  preserved 
*  LOG.  cit.,  p.  437. 


DISCONTINUOUS  VARIATION.  63 

for  Bowing  the  following  year.  The  proportion  of  four 
leaflet  plants  steadily  increased,  and  in  the  fourth  gen- 
eration the  most  widely  diverging  plants  had  the  fol- 
lowing (percentage)  proportions  of  leaves  with  from 
3  to  7  leaflets. 


TOTAL  NUMBER  OF 

LEAVES  COUNTED. 

Number  of  leaflets, 

3 

4 

5 

6 

7 

Normal  plant, 

17 

16 

37 

14 

16 

172 

Atavistic    " 

75 

19 

5 

0 

1 

216 

Extreme  variation,         12         9        22        17      40  97 

The  plant  considered  as  "  normal "  was  obviously  of 
a  five  leaflet  type,  the  numbers  of  leaves  with  3  and  4 
leaflets,  and  those  with  6  and  7  leaflets,  being  dis- 
tributed symmetrically  around  it.  A  comparison  of  the 
numbers  of  leaflets  in  the  "  atavistic  plant  "  with  those 
in  the  "  extreme  variation  "  is  interesting,  as  showing 
the  range  of  variation  possible  in  plants  of  the  same 
stock. 

As  an  instance  of  the  formation  of  a  variety,  De 
Vries'  experiments  with  Chrysanthemum  segetum 
grandiflorum  may  be  quoted.*  Starting  in  1896  with 
plants  which  had  21  ray  florets  occurring  most  fre- 
quently in  their  capitula,  and  none  of  which  had  more 
than  23  florets,  he  picked  out  each  year  the  two  or 
three  plants  richest  in  florets  for  breeding  with,  and 
sowed  their  seed  the  following  year.  In  1897  a  single 
flower  was  obtained  having  34  florets,  but  the  21  floret 
form  was  still  the  commonest.  In  1898  one  of  the 
flowers  had  48  florets,  the  commonest  forms  now  having 
26  or  34  florets.  In  1899  one  had  67  florets,  the  com- 
monest forms  having  26  or  33  to  35  florets,  and  in  1900 
*Loc.  cit.,p.  523. 


64  DISCONTINUOUS  VAEIATION. 

one  had  101  florets,  the  commonest  form  having  47 
florets. 

Let  us  now  return  to  the  cases  of  dimorphism  men- 
tioned at  the  beginning  of  the  chapter.  Instances  were 
there  adduced  in  which  the  dimorphism  was  slight, 
fairly  marked,  or  so  great  that  the  two  forms  scarcely 
overlapped  at  all.  To  what  may  such  dimorphism  be 
due?  Bateson  points  out  that  a  dimorphic  condition 
may  have  arisen  from  a  previous  monomorphic  one,  or 
it  may  always  have  been  present  since  the  character  was 
first  acquired.  As  already  stated,  the  first  view  is  the 
one  which  finds  favour  in  the  eyes  of  most  biologists, 
but  on  the  other  hand  there  is  a  certain  amount  of  evi- 
dence to  show  that  the  second  view  may  hold  good,  at 
least  in  some  cases.  It  is  a  well-known  fact  that  when 
two  breeds  are  crossed,  their  characters  do  not  always 
blend,  but  are  transmitted  in  an  unmodified  state  to  the 
offspring  from  one  or  from  both  parents.  For  instance, 
in  breeding  game  fowls,  if  one  crosses  a  black  with  a 
white  game,  birds  of  both  breeds  of  the  clearest  colour 
are  obtained.  "  Sir  R.  Heron  crossed  during  many 
years  white,  black,  brown,  and  fawn-coloured  Angora 
rabbits,  and  never  once  got  these  colours  mingled  in  the 
same  animal,  but  often  got  all  four  colours  in  the  same 
litter."  *  Again,  Miss  E.  A.  Saunders  f  has  recently 
made  observations  on  Biscutella  lavigata,  a  cruciferous 
plant  occurring  as  a  perennial  herb  in  the  alpine  and 
sub-alpine  regions  of  middle  and  southern  Europe.  It 
was  observed  by  Bateson  that  this  species  exhibits  two 
distinct  forms,  which  exist  side  by  side,  the  one  hairy 

*  "  Animals  and  Plants,"  vol.  ii.  p.  70. 
fProc.  Roy.  Soc.,  vol.  Ixii.  p.  11. 


DISCONTINUOUS  VARIATION.  65 

and  the  other  smooth  or  glabrous.  Intermediate  forms 
were  also  found,  but  these  were  scarce.  Some  of  the 
ripe  seeds  of  these  plants  were  obtained,  and  grown  in 
England.  A  portion  of  the  seedlings  were  derived 
from  cross-fertilised  seeds  of  known  origin,  and  it  was 
found  that  though  there  was  to  a  certain  extent  a  blend- 
ing of  parental  characters  as  regards  hairiness  and 
smoothness  in  the  offspring  of  plants  of  dissimilar  types, 
giving  rise  to  intermediate  forms,  yet  this  intermediate 
condition  was  found  only  quite  exceptionally  among 
full-grown  individuals.  It  was  much  more  common  in 
the  young  plants,  but  as  these  grew  older,  their  leaves 
became  smooth,  and  hence  almost  all  the  plants  were 
ultimately  either  hairy  or  glabrous;  that  is  to  say,  they 
varied  discontinuously. 

Supposing  a  dimorphic  condition  is  due  to  internal 
causes,  or  to  the  fact  that  it  is  the  nature  of  the  plant 
to  vary  in  this  way  around  two  "  positions  of  organic 
stability,"  as  Galton  has  termed  them,  rather  than 
around  one  such  position,  then  it  would  seem  almost 
impossible  to  get  further  to  the  root  of  such  causes. 
Supposing,  on  the  other  hand,  as  is  probably  true  in  the 
majority  of  instances,  this  dimorphic  condition  has  been 
derived  from  a  previous  monomorphic  one,  then  we 
may  hold  a  more  reasonable  hope  of  being  able  to  eluci- 
date the  cause  or  causes  of  this  evolution  from  one  con- 
dition to  another.  The  problem  of  the  splitting  up  of 
species  was  recognised  by  Darwin  to  be  one  of  immense 
importance,  and  he  discussed  it  at  some  length  in  the 
"  Origin  of  Species."  *  The  chief  cause  of  divergence 
of  character  he  attributed  to  the  circumstance  that 
*Ed.  vi.  p.  86. 


66  DISCONTINUOUS  VARIATION. 

"  the  more  diversified  the  descendants  from  any  one 
species  become  in  structure,  constitution,  and  habits,  by 
so  much  the  more  will  they  be  better  enabled  to  seize 
on  many  and  widely  diversified  places  in  the  polity  of 
nature,  and  so  be  enabled  to  increase  in  numbers." 
He  also  attached  considerable  importance  to  geograph- 
ical isolation  of  a  portion  of  a  species,  as  an  element  in 
the  modification  of  species  through  natural  selection. 

Though  Darwin's  principle  of  diversification  of 
structure  is  doubtless  a  very  true  one,  yet  it  does  not  in 
itself  contain  sufficient  clue  as  to  why  a  species  should 
split  up  into  two  or  more  varieties.  Thus,  if  by  some 
means  these  actually  arose,  but  both  continued  to  in- 
habit the  same  area,  it  is  difficult  to  understand  why 
intercrossing  should  not  rapidly  reduce  them  to  the 
single  species  from  which  they  took  their  origin.  It 
was  to  overcome  this  difficulty  that  Romanes  suggested 
his  theory  of  "  Physiological  Selection."  *  This  theory 
is  founded  on  the  fact  that  individuals  of  a  species, 
though  fertile  with  some,  may  be  perfectly  sterile  with 
other  individuals,  and  this  apparently  independent  of 
any  differences  of  form,  colour,  or  structure.  Romanes 
thought  that  this  incompatibility  might  run  through  a 
whole  race  or  strain,  and  so  a  group  of  individuals  of  a 
species  be  in  a  physiological  sense  isolated  from  the  rest, 
and  therefore  able  to  vary  independently,  without  hav- 
ing their  newly  acquired  characteristics  swamped  by 
intercrossing.  As  Wallace  has  very  clearly  shown,  t 
this  theory,  in  the  form  originally  proposed  by  its  au- 

*Journ.  Linn.  Soc.  (ZoSl.),  vol.  xix.  p.  337,  1886.    Also   "Dar- 
win and  after  Darwin,"  vol.  iii.  p.  41. 
f"  Darwinism,"  p.  181. 


DISCONTINUOUS  VARIATION.  67 

thor,  cannot  stand,  at  least  for  members  of  the  Animal 
Kingdom  in  which  there  is  no  promiscuous  union  of  the 
sexes.  For  instance,  if  10  per  cent,  of  the  members  of 
a  species  are  thus  physiologically  isolated,  so  as  to  be 
fertile  inter  se,  but  sterile  when  crossed  with  any  of  the 
other  members  of  the  species,  and  if  the  interbreeding 
take  place  purely  according  to  the  laws  of  chance,  then 
on  an  average  only  one-tenth  of  these  10  per  cent, 
will  happen  to  pair  with  individuals  with  which  they  are 
fertile,  and  the  remaining  nine-tenths  will  form  abso- 
lutely sterile  unions.  Thus  this  physiologically  isolated 
section  will  never  be  able  to  increase  in  numbers  and 
establish  itself.  In  the  case  of  flowering  plants  which 
are  fertilised  by  insects,  each  of  which  perhaps  visits 
ten  or  more  flowers  in  a  journey,  Fletcher  Moulton  has 
shown  *  that  Wallace's  objection  does  not  hold,  as  the 
dimunition  in  fertility  in  such  a  case  is  practically  neg- 
ligeable.  In  his  more  recent  discussion  of  the  theory 
Romanes  somewhat  modified  his  views,  and  laid  more 
stress  on  the  fact  that  the  mutual  sterility  may  have 
been  slight  at  first,  and  have  been  subject  to  a  gradual 
development,  it  acting  as  a  segregating  cause  in  a  de- 
gree proportional  to  its  completeness.  However,  he 
makes  no  suggestion  as  to  why  and  how  such  physio- 
logical incompatibility  should  arise,  other  than  general 
ones  such  as  the  influence  of  food  and  climate,  and 
spontaneous  variability  of  the  reproductive  system. 

It   seems   to   me   that   the   origin   of   this    physio- 
logical   barrier    which    so    generally    exists    between 
species     can     be    most    readily     accounted     for    by 
assuming  that  in  some  cases  at  least  it  is,  as  Romanes 
*  "  Darwin  and  after  Darwin,"  vol.  iii.  p.  165. 


68  DISCONTINUOUS  VARIATION. 

suggests,  slowly  evolved  from  an  originally  almost 
imperceptible  degree  of  infertility,  but  that  this  takes 
place  only  simultaneously  with  the  evolution  of  mor- 
phological character,  in  consequence  of  some  form  of 
isolation.  Thus,  suppose  a  number  of  individuals 
of  a  species  become  for  a  time  separated  from  the 
remainder  of  the  species  by  a  geographical  barrier, 
by  migration,  or  some  other  cause  of  isolation,  whereby 
they  are  enabled  to  vary  independently  of  the  gen- 
eral stock  in  response  to  changed  conditions  of  life. 
Then  as  they  gradually  become  more  and  more  diver- 
gent from  the  parent  stock  in  respect  of  morphological 
characters,  it  is  highly  probable  that  they  may  concur- 
rently— perhaps  from  the  direct  action  of  the  body 
tissues  on  the  reproductive  system — diverge  also  in  re- 
spect of  physiological  characters.  Should  any  of  them 
now  happen  to  meet  and  intercross  with  individuals  of 
the  parent  stock,  or  even  if  they  should  occupy  the 
same  breeding  area  again,  their  newly  acquired  mor- 
phological characters  would  no  longer  be  in  danger  of 
being  swamped,  for  the  simple  reason  that  few  or  no 
hybrid  offspring  would  result  from  such  crossing.  In 
the  case  of  the  higher  animals,  also,  it  is  probable  that 
individuals  of  different  varieties  or  sub-species,  once 
these  are  formed,  instinctively  tend  to  breed  amongst 
themselves,  and  hence  the  chance  of  production  of  hy- 
brid offspring  is  still  further  diminished.  Thus  Dar- 
win records  *  that  in  Paraguay  it  is  believed  "  that  the 
native  horses  of  the  same  colour  and  size  prefer  asso- 
ciating with  each  other,  and  that  the  horses  which  have 
been  imported  from  Entre  Eios  and  Banda  Oriental 
*  Ibid.,  vol.  ii.  p.  80. 


DISCONTINUOUS  VARIATION.  69 

into  Paraguay  likewise  prefer  associating  together." 
Again,  "  It  has  been  observed,  in  a  district  stocked  with 
heavy  Lincolnshire  and  light  Norfolk  sheep,  that  both 
kinds,  though  bred  together,  when  turned  out,  in  a  short 
time  separate  to  a  sheep."  Still  again,  with  respect  to 
fallow-deer,  "  Mr.  Bennett  states  that  the  dark  and  pale 
coloured  herds,  which  have  long  been  kept  together  in 
the  Forest  of  Dean,  in  High  Meadow  Woods,  and  in  the 
New  Forest,  have  never  been  known  to  mingle."  Dar- 
win adduces  other  similar  instances,  in  the  case  of  the 
dog,  horse,  sheep,  rabbit,  and  pigeon]  hence  there  can 
be  little  doubt  of  the  genuineness  of  the  phenomenon, 
even  though  it  is  not  based  on  very  exact  observation. 

Supposing  that  the  above  view  is  correct,  it  follows 
that  between  at  least  some  varieties  there  must  exist  a 
greater  or  less  degree  of  sterility.  Of  course  it  is  not 
necessary  that  divergence  of  morphological  character 
should  always  be  accompanied  by  corresponding  diver- 
gence of  physiological  character;  but  merely  that  this 
is  sometimes  the  case.  Upon  this  point  Darwin  has  col- 
lected a  considerable  amount  of  evidence  in  his  "  Ani- 
mals and  Plants."  *  One  or  two  of  the  cases  there 
cited  may  be  quoted  here.  Gartner  found  that  a 
variety  of  dwarf  maize,  bearing  yellow  seed,  showed  a 
considerably  diminished  fertility  with  a  tall  maize  bear- 
ing red  seed,  though  both  varieties  were  perfectly  fer- 
tile when  crossed  inter  se.  Again,  in  the  genus  Ver- 
bascum,  numerous  experiments  were  made  by  Gartner 
with  the  white  and  yellow  varieties  of  V.  lychnitis  and 
V.  blatteria,  when  he  found  that  crosses  between  simi- 
larly coloured  flowers  yielded  more  seed  than  those  be- 
*Vol.  ii.  p.  82. 


70  DISCONTINUOUS  VARIATION. 

tween  dissimilarly  coloured  ones.  These  experiments 
have  been  repeated  and  extended  by  Scott  with  con- 
firmatory results. 

Still  better  evidence  than  that  quoted  by  Darwin  has 
been  obtained  by  Jordan  in  a  laborious  research  on 
various  species  of  plants — annuals  and  perennials,  bul- 
bous and  aquatic,  trees  and  shrubs — extending  over 
thirty  years.  Jordan  found  that  when  a  Linnean 
species  is  indigenous  to  a  country,  and  is  of  common 
occurrence,  it  is  represented  by  more  or  less  numerous 
and  perfectly  constant  varieties,  all  growing  in  intimate 
association  with  one  another.  It  was  found  that  in 
many  hundreds  of  cases  these  varieties,  though  they 
differed  but  slightly  in  morphological  characters,  came 
true  to  seed,  but  were  always  more  or  less  infertile  when 
crossed  inter  se* 

With  regard  to  members  of  the  Animal  Kingdom, 
there  is  very  little  evidence  indeed.  The  following 
anthropological  data  may  perhaps  be  held  valid.  From 
statistics  collected  in  Prussia  between  1875  and  1890, 
it  was  found  that  Protestants,  Catholics,  and  Jews, 
when  marrying  among  themselves,  had,  on  an  average, 
respectively  4.35,  5.24,  and  4.21  children.  When,  how- 
ever, the  husband  was  a  Jew  and  the  wife  a  Protestant 
or  Catholic,  the  numbers  of  children  were  only  1.58  and 
1.38  respectively:  and  when  the  wife  was  a  Jewess  and 
the  husband  a  Protestant  or  Catholic,  only  1.78  and 
1.66  respectively. t  Whether  this  apparent  partial 
sterility  was  due  to  differences  of  race  or  to  social  rea- 
sons, it  is  impossible  to  say.  Again,  Professor  Broca  t 

*  Quoted  from  Romanes,  ibid.,  p.  86. 

f  Quoted  from  Mayo  Smith's  "  Statistics  of  Sociology,"  p.  115. 

\  "  On  the  Phenomena  of  Hybridity  in  the  Genus  Homo,"  1864. 


DISCONTINUOUS  VARIATION.  71 

has  brought  forward  evidence  that  some  races  of  man 
show  diminished  fertility  together. 

In  order  to  obtain  further  evidence,  the  author  * 
made  numerous  observations  on  the  effects  of  crossing 
the  colour  varieties  of  the  sea-urchins  Splicer  echinus 
granularis  and  Strongylocentrotus  lividus.  With  the 
former  organism  the  numbers  of  blastulse  and  of  larvae 
produced  on  crossing  dissimilar  colour  varieties  were 
distinctly  smaller  than  for  similar  varieties.  In  the 
most  marked  instance,  the  similar  color  varieties  yielded 
on  an  average  98.5  per  cent,  of  blastulse,  and  73  per 
cent,  of  larvaa,  whilst  the  dissimilar  yielded  68  per  cent, 
of  blastulaa  and  only  15.6  per  cent,  of  larvaa.  More- 
over these  latter  larvaa  were  4.5  per  cent,  smaller  than 
the  others.  In  the  case  of  Strongylocentrotus,  however, 
where  the  colour  varieties  are  much  less  pronounced, 
there  was  very  little  difference  of  fertility. 

There  can  be  no  doubt,  therefore,  that  certain  varie- 
ties show  a  greater  or  less  degree  of  mutual  infertility, 
though  this  is  doubtless  not  nearly  so  marked,  or  of 
such  frequent  occurrence,  as  in  the  case  of  species. 
Whatever  view  be  taken  as  to  the  cause  of  such  infer- 
tility, and  its  relation  to  divergent  evolution,  these  in- 
stances quoted  have  also  an  intrinsic  value.  They  show 
that  just  as  the  deviations  from  the  average  in  respect 
of  morphological  characters  may  form  double  humped 
curves,  so  the  deviations  in  respect  of  physiological 
characters  may  show  corresponding  irregularities, 
though  of  course  it  is  impossible  to  measure  them 
exactly  and  construct  their  curves  of  variation. 

*  Phil.  Trans.  1898,  B.  p.  511. 


CHAPTER  III. 

CORRELATED    VARIATIONS. 

The  measurement  of  correlation — Gallon's  function — Correlation 
between  various  organs  in  man,  in  local  races  of  the  shrimp,  and  in 
crabs — Comparison  between  primitive  and  civilised  races  of  man — 
Correlation  between  morphological  characters  and  the  reproduct- 
ive system — Genetic  Selection  in  man — Especial  fertility  of  type 
forms  in  certain  plants — Evolution  in  the  Peppered  moth — Paral- 
lel variation — Importance  of  mathematical  treatment  of  variation. 

ALL  parts  of  an  organism  are  to  a  certain  extent  re- 
lated to  each  other,  so  that  when  one  part  varies  other 
parts  vary  simultaneously  in  a  greater  or  less  degree. 
That  is  to  say,  variations  are  correlated.  The  most 
marked  and  obvious  correlation  is  that  existing  between 
homologous  parts.  The  symmetry  of  the  correspond* 
ing  or  homologous  organs  on  the  right  and  left  sides  of 
the  body,  which  is  present  in  most  animals,  represents 
a  very  close  degree  of  correlation.  But  even  in  this 
case  the  correlation  is  not  constant  or  complete.  Thus 
the  two  arms  and  the  two  legs  of  a  man  resemble  each 
other  very  closely  indeed,  but  careful  measurement 
shows  that  the  resemblance  is  not  absolute.  Again,  the 
arms,  as  a  rule,  vary  in  length  more  or  less  in  the  same 
proportion  as  the  legs,  but  personal  experience  will 
probably  recall  instances  to  the  contrary,  in  which  the 
length  of  the  limbs  was  quite  disproportionate.  Be- 
tween the  arms  and  the  legs,  therefore,  the  degree  of 

72 


CORRELATED  VARIATIONS.  73 

correlation  is  obviously  less  close  than  between  arm  and 
arm,  or  leg  and  leg.  Still  again,  personal  experience 
teaches  us  that  there  is  correlation  between  even  the 
length  of  the  face  and  that  of  the  limbs.  Tall  men  as 
a  rule  have  longer  faces  than  short  men;  or,  a  more 
striking  instance,  greyhounds  have  long  heads  and  long 
legs  on  the  one  hand,  as  compared  with  bull-dogs  with 
short  heads  and  short  legs  on  the  other.  Between  the 
length  of  face  and  length  of  limb,  however,  it  is  clear 
that  there  is  a  less  degree  of  correlation  than  between 
length  of  arm  and  of  leg,  and  between  certain  other 
organs  of  the  body  the  connection  must  be  less  intimate 
still.  It  follows,  therefore,  that  between  the  various 
parts  there  may  exist  all  degrees  of  correlation,  stretch- 
ing from  an  almost  perfect  degree  of  resemblance  down 
to  an  absolute  lack  of  it.  We  must  also  recognise  the 
existence  of  negative  correlation,  in  which  the  variation 
of  one  part  in  one  direction  is  accompanied  by  a  greater 
or  less  degree  of  variation  of  another  part  in  the  oppo- 
site direction.  Here  again  we  may  experience  all  de- 
grees of  negative  correlation,  just  as  of  positive.  In- 
stances of  negative  correlation  are  much  less  frequent 
than  those  of  positive,  and  the  only  one  known  to  me  in 
the  case  of  man  is  that  recently  discovered  by  Professor 
Pearson.*  It  was  found  that  between  stature  and 
head  index  the  correlation  is  distinctly  negative,  or 
that  brachycephalic  or  relatively  broad-headed  persons 
are  slightly  shorter  than  dolichocephalic  or  narrow- 
headed. 

From  what  has  been  said  it  is  clear  that  a  bald  state- 
ment that  in  such  and  such  a  case  one  part  or  organ  is 
*Proc.  Roy.  Soc.,  Ixvi.  p.  23, 1900. 


74  CORRELATED  VARIATIONS. 

correlated  with  another  conveys  no  exact  meaning. 
Such  a  statement  must  vary  according  to  the  notion  of 
the  observer  as  to  what  does  and  what  does  not  consti- 
tute correlation.  In  order  to  obtain  reliable  and  com- 
parable data  concerning  the  degree  of  correlation,  it  is 
necessary  to  obtain  a  mathematical  expression  for  it, 
just  as  one  was  found  to  be  necessary  for  indicating  the 
range  of  a  variation.  The  fundamental  theorems  of 
correlation  were  for  the  first  time  exhaustively  discussed 
by  Bravais  *  more  than  half  a  century  ago,  but  a  more 
convenient  and  improved  method  of  obtaining  an  ex- 
pression was  first  indicated  by  Galton,  and  he  termed  it 
the  correlation  constant,  or  r.  It  is  now  more  generally 
known  as  "  Galton's  function." 

The  principle  on  which  this  constant  is  determined  is 
best  explained  by  a  concrete  instance,  viz.,  one  given  by 
Galton  in  the  original  paper  in  which  he  explained  his 
method.f  Galton's  data  are  anthropometric  ones,  ob- 
tained at  his  own  laboratory,  and  consist  of  several 
measurements  made  on  350  males  of  21  years  and  up- 
wards. For  instance,  Galton  found  that  the  average 
relation  between  stature  and  cubit,  or  distance  between 
the  elbow  of  the  bent  arm  and  the  tip  of  the  middle 
finger,  was  as  100  to  3Y.  In  determining  the  correla- 
tion between  these  two  measurements,  however,  it  is 
obvious  that  it  is  not  possible  to  compare  the  absolute 
amount  of  variation  of  the  one  with  the  absolute  amount 
of  the  other,  or  even  the  proportionate  amounts,  but 

*  "Analyse  mathematique  sur  les  probabilites  des  erreurs  de  situ- 
ation d'un  point."  Memoires  par  divers  Savans,  T.  ix.,  Paris, 
1846,  p.  255. 

fProc.  Roy.  Soc.,  vol.  xlv.  p.  135,  1888. 


CORRELATED  VARIATIONS.  75 

we  must  first  transmute  them  into  units  dependent  on 
their  respective  scales  of  variability.  We  shall  thus 
cause  a  long  or  a  short  cubit  and  an  equally  long  or 
short  stature,  as  compared  to  the  general  run  of  cubits 
and  statures,  to  be  designated  by  identical  scale  values. 
The  most  convenient  unit  to  employ  is  the  value  of  the 
probable  error  of  each  group.  The  probable  error  of 
the  cubit  is  .56  inch  =  1.42  cm.;  and  of  the  stature, 
1.75  inch  =  4.44  cm.  Therefore  each  of  the  measure- 
ments of  the  cubit  must  be  transmuted  into  terms  of  a 
new  scale,  in  which  each  unit  =  .56  inch,  and  each  of 
the  measurements  of  the  stature  into  those  in  which 
each  unit  —  1.75  inch.  After  this  has  been  done,  we 
shall  find  that  on  an  average  each  deviation  in  the 
stature  of  say  1  unit  from  the  mean  is  not  accompanied 
by  a  similar  deviation  of  1  unit  in  the  cubit,  but  by 
only  .8  of  a  unit.  Conversely  it  is  found  that  in  a 
similar  manner  each  deviation  in  the  cubit  of  1  unit 
from  the  mean  is  accompanied  by  only  .8  of  a  unit  of  de- 
viation in  the  stature.  The  degree  of  correlation,  or  r, 
between  the  one  organ  and  the  other,  is  therefore  said 
to  be  .8.  If  the  correlation  had  been  perfect,  then  this 
r  would  have  been  equal  to  1,  and  if  it  had  been  entirely 
wanting,  then  it  would  have  been  0.  Comparison  with 
other  data  shows  that  a  correlation  of  .8  is  a  high  one, 
not  often  surpassed.  The  other  correlation  constants 
determined  by  Galton  are  the  following: 

MEAN  r. 

Stature  and  head  length,  35 

Stature  and  middle  finger, 7 

Middle  finger  and  cubit,  85 

Head  length  and  head  breadth, 45 

Stature  and  height  of  knee,        .....        .9 

Cubit  and  height  of  knee, 8 


76  CORRELATED  VARIATIONS. 

Here  we  see  that  the  maximum  amount  of  correlation 
was  observed  between  stature  and  height  of  knee,  and 
the  minimum  between  stature  and  head  length.  Even 
in  this  latter  instance,  however,  the  correlation  was 
fairly  marked.  Thus  a  constant  of  .35  indicates  that 
in  men  1.75  inch,  or  1  unit,  above  the  mean  stature,  the 

QH 

length  of  head  will  on  an  average  be  ^-^  X  .19  =.0665 

-L .  UU 

inch  above  the  mean,  .19  inch  being  the  probable  error 
of  variation  of  the  head  length.  The  height  of  knee, 
on  the  other  hand,  would  on  an  average  be  no  less  than 

Q 

:p-r  X  .80  =  .72  inch  greater.     The  various  medians  or 

middlemost  values  and  probable  errors  found  by  Gal- 
ton  are  as  follows : 

MEDIAN.  PROBABLE  ERROR. 

DIMENSION.  INCH.  CENTIM.  INCH.          CBNTIM. 

Head  length,  7.62  19.35  .19  .48 

Head  breadth,  6.00  15.24  .18  .46 

Stature,  67.20  170.69  1.75  4.44 

Left  middle  finger,  4.54  11.53  .15  .38 

Left  cubit,  18.05  45.70  .56  1.42 

Height  of  rt.  knee,  20.50  52.00  .80  2.03 

In  order  to  determine  the  degree  of  correlation  be- 
tween any  two  organs,  it  is  therefore  necessary  to  adopt 
the  following  procedure.  Sort  out  all  the  individuals 
into  groups  such  as,  for  instance,  in  the  case  of  stature, 
those  varying  from  64  to  65,  65  to  66,  66  to  67  inches, 
and  so  on,  and  then  determine  in  each  of  these  groups 
the  mean  of  all  the  deviations  from  the  average  of  the 
organ  of  which  the  correlation  with  stature  is  to  be 
determined.  Thus,  in  the  group  of  individuals  64  to 
65  inches  high,  the  average  difference  of  all  the  indi- 
vidual cubit  measurements  from  the  median  of  the 


CORRELATED  VARIATIONS.  77 

cubit  (18.05  inches),  is  about  .6  inch.  Let  the  devia- 
tion of  each  value  for  stature  from  its  median  (67.20 
inches)  be  now  divided  by  the  probable  error  of  varia- 
tion of  stature  (i.  e.,  1.75  inch),  and  each  associated 
mean  deviation  of  cubit  be  divided  by  its  probable  error 
(i.  e.,  .56  inch).  Then,  by  dividing  each  of  these  terms 
for  cubit  by  the  corresponding  term  for  stature,  a  series 
of  values  is  obtained,  each  representing  the  amount  of 
correlation  between  the  various  degrees  of  stature  and 
the  cubit.  These  values  would  be  approximately  equal 
in  amount  if  a  very  large  number  of  observations  were 
made,  but  with  only  moderate  numbers  they  vary  very 
considerably.  A  mean  of  all  of  them  may  be  called  r, 
or  the  average  degree  of  correlation  between  cubit  in 
relation  to  stature.  In  a  similar  manner  the  individuals 
must  be  split  up  into  groups  in  respect  of  cubit,  and  the 
associated  deviation  of  stature  determined.  Another 
series  of  correlation  values  will  be  obtained,  represent- 
ing stature  in  relation  to  cubit,  of  which  the  mean  may 
be  called  ra.  This  value  is  found  to  be  approximately 
equal  to  rl}  and  the  mean  of  r^  and  ra  is  called  r,  or 
the  correlation  constant. 

The  degree  to  which  these  individual  correlation 
values  vary  is  best  shown  by  means  of  a  diagram.  The 
one  given  in  Fig.  16  is  taken  from  Professor  Weldon's 
paper  on  correlated  variations  in  Crangon  vulgaris* 
and  represents  the  correlation  between  the  post-spinous 
carapace  length  and  the  total  carapace  length  in 
Plymouth  shrimps. 

The  mean  value  of  r  found  was  .81.  In  this  dia- 
gram, the  deviations  of  the  organ  whose  value  is  fixed 
*Proc.  Roy.  Soc.,  li.  p.  2,  1892. 


78 


CORRELATED  VARIATIONS. 


are  measured  along  the  ordinates,  they  varying  on  an 
average  between  the  extremes  of  +  3.19  and  —  3.01, 
whilst  the  mean  deviations  of  the  associated  organ  are 


+  2 


*1 


-1 


-9 


2 


-1 


+  2 


+  2 


-1 


-S 


•piQ.  16.— Correlation   between  post-spinous  carapace   length  and 
total  carapace  length  of  shrimp. 

measured  along  the  abscissae,  they  varying  on  an  aver- 
age from  +  2.92  to  —  2.17.  The  crosses  in  the  dia- 
gram indicate  the  values  obtained  when  the  carapace 
length  was  fixed,  and  the  circles  those  when  the  post- 


CORRELATED  VARIATIONS.  79 

spinous  portion  was  fixed.  The  line  drawn  through 
them  indicates  the  ratio  .81.  Every  point  should 
theoretically  lie  on  it,  and  it  will  be  seen  that  they  do, 
as  a  matter  of  fact,  lie  very  closely  around  it. 

This  correlation  constant  of  .81  was  obtained  by 
Professor  Weldon  for  shrimps  collected  at  a  particular 
locality,  viz.,  Plymouth.  Similar  determinations  were 
also  made  for  shrimps  obtained  from  other  localities, 
with  the  following  results : 

In  Plymouth  r  —  .81  (1000  individuals  examined) 

In  Southport  .85  (  800           "              "         ) 

In  Roscoff  .80  (  500           "              "         ) 

In  Sheerness  .85  (  380           "              "         ) 

In  Helder  .83  (  300           "              "        ) 

The  approach  to  identity  between  these  values  is  very 
striking,  the  differences  appearing  to  be  within  the 
probable  error  of  each  determination.  There  seems  a 
reasonable  ground  for  assuming,  therefore,  that  the 
degree  of  correlation  between  the  two  particular  organs 
measured  is  practically  constant  in  all  the  races  exam- 
ined. The  correlation  between  other  organs  was  also 
estimated,  but  this  was  in  each  instance  very  much 
slighter,  and  in  the  case  of  the  telson  and  sixth  abdom- 
inal tergum,  it  was  negative.  Considering  the  degree 
of  independence  of  these  organs,  as  shown  by  the  small- 
ness  of  their  correlation  constants,  the  similarity  be- 
•tween  the  values  for  the  two  local  races  is  probably  as 
close  as  could  be  expected.  Hence,  as  both  the  organs 
measured  and  the  samples  of  shrimps  examined  were 
chosen  by  chance,  any  result  which  holds  for  all  these 
organs  through  all  these  races  may  be  reasonably  ex- 
pected to  prove  generally  true  of  all  organs  through  the 
whole  species. 


80  COERELATED  VARIATIONS. 


CARAPACE  LENGTH  CARAPACE  LENGTH  TELSON  AND 

AND  TERGUM  VI.        AND  TELSON.  TERGUM  VI. 

Plymouth,                 .09                            .18  -.11 

Southport,                  .06                             .14  -.09 


Professor  Weldon  points  out  that  the  above  results 
lead  us  to  hope  that  it  may  be  possible  to  determine  con- 
stants for  any  species  of  animal  which  would  "  give  an 
altogether  new  kind  of  knowledge  of  the  physiological 
connection  between  the  various  organs  of  animals,  while 
a  study  of  those  relations  which  remain  constant 
through  large  groups  of  species  would  give  an  idea,  at- 
tainable at  present  in  no  other  way,  of  the  functional 
correlations  between  various  organs  which  have  led  to 
the  establishment  of  the  great  sub-divisions  of  the  ani- 
mal kingdom." 

In  a  subsequent  paper,*  Professor  Weldon  deter- 
mined no  less  than  23  different  correlation  constants, 
between  various  pairs  of  organs  in  1000  adult  female 
crabs  (Carcinus  mcenas),  collected  in  Plymouth  Sound, 
and  in  another  1000  collected  in  the  Bay  of  Naples. 
He  found  that  there  was  as  a  rule  a  remarkable  degree 
of  correspondence  between  the  values  of  r  derived  from 
an  investigation  of  the  same  pair  of  organs  in  the  two 
races  examined.  There  were  in  some  cases  consider- 
able differences  between  the  values,  it  is  true,  but  Wel- 
don considers  that  these  were  in  no  case  sufficient  to 
justify  the  assertion  that  the  degree  of  correlation  is 
really  different  in  the  two  cases.  It  should  be  men- 
tioned, however,  that  Professor  Pearson  f  does  not 

*Proc.  Roy.  Soc.,  liv.  p.  318. 
f  Phil.  Trans.  1896,  A.  p.  267. 


CORRELATED  VARIATIONS.  81 

agree  with  this  conclusion,  but  thinks  that  the  differ- 
ences observed  are  too  large  to  justify  such  an  assump- 
tion. 

At  Professor  Weldon's  suggestion,  Mr.  E.  Warren  * 
undertook  similar  measurements  on  2300  specimens  of 
another  crab,  Portunus  depurator,  also  obtained  from 
Plymouth.  The  accompanying  table  gives  the  results 
obtained  by  Warren,  and  some  of  those  obtained  by 
Weldon: 

C.   MCENAS,          PORTUNUS 

C.  MOENAS,        PLYMOUTH     DEPURATOR, 

ORGANS.  NAPLES  RACK.          RACE.  PLYMOUTH. 

Total  breadth  and  frontal  breadth,          .08  .10  .14 

R.  antero-lateral,           .66  .65  .67 

"          "           R.  dentary  margin,        .50  .55  .56 

Frontal  breadth  and  R.  antero-lat.,          .29  .24  .30 

"          "            R.  dentary  margin,  — .23  —.18  —.03 

L.  dentary  margin,  — .26  —.20  —.01 

R.  antero-lateral  and  L.  antero-lat.,         .76  .78  .86 

R.  dentary  margin,     .71  .78  .80 

"           "         L.  dentary  margin,     .60  .70  .74 

On  glancing  through  this  table,  it  will  be  seen  that, 
with  two  exceptions,  the  values  for  the  two  races  of 
C.  mcenas  differ  from  one  another  nearly  as  much  as 
they  do  from  the  constants  of  Portunus,  an  animal  be- 
longing to  a  different  genus.  It  is  probable,  however, 
that  the  larger  differences  in  this  latter  animal  do  indi- 
cate real  differences  in  the  correlation  constants,  asso- 
ciated, perhaps,  with  changes  in  habit  or  environment. 
For  example,  it  is  conceivable  that  a  crab  which  swims 
might  find  it  advantageous  to  be  more  symmetrical  than 
one  which  only  crawls  between  the  tide  marks.  Por- 
tunus does  swim  to  a  certain  extent,  and  we  see  that 
the  correlation  of  the  two  sides  of  its  body  is  greater 
than  in  the  essentially  shore-living  Carcinus. 
*Proc.  Roy.  Soc.,  Ix.  p.  221. 


82  COREELATED  VAEIATIONS. 

Another  most  laborious  research  undertaken  at  Pro- 
fessor Weldon's  suggestion  is  that  of  H.  Thompson,* 
on  the  correlation  of  certain  external  parts  of  the 
prawn,  Palcemon  serratus.  Twenty-two  measurements 
were  made  on  1000  adult  females,  and  from  these  the 
value  of  Galton's  function  was  calculated  for  56  pairs 
of  organs.  As  might  be  expected,  the  degree  of  corre- 
lation was  highest  between  the  paired  organs;  e.  g.,  .94 
between  the  right  and  left  squames.  Also  there  was  a 
strong  correlation  between  the  terga  of  adjacent  ab- 
dominal segments,  their  values  ranging  between  .58 
and  .71. 

Of  other  recent  work  on  correlation,  that  by  Miss 
Lee  and  Professor  Pearson  f  may  be  briefly  alluded  to. 
This  consists  in  a  comparison  of  measurements  on  cer- 
tain long  bones  of  about  40  male  and  25  female  skele- 
tons of  the  Aino  race  (a  primitive  tribe  dwelling  in 
Japan),  with  corresponding  measurements  of  50  male 
and  50  female  skeletons  of  the  modern  French  race. 
It  was  found  that  the  transition  from  the  uncivilised  to 
the  civilised  condition  is  accompanied  by  well-marked 
changes  in  the  sexual  relationships;  primitive  man  and 
woman  being  more  nearly  equal  in  size,  variability,  and 
correlation  than  highly  civilised  man  and  woman. 
Civilised  man  has  gained  in  size  on  woman,  but  this  has 
been  accompanied  by  a  relative  loss  in  variability  and 
the  correlation  of  parts.  The  general  result  of  in- 
creased civilisation  is  to  increase  the  absolute  size  and 
amount  of  variation.  In  females,  also,  the  degree  of 
correlation  is  increased,  but  in  males  this  remains  sta- 

*  Proc.  Roy.  Soc. ,  Iv.  p.  234. 
fProc.  Roy.  Soc.,  Ixi.  p.  343. 


CORRELATED  VARIATIONS.  83 

tionary.  It  is  therefore  impossible  to  say  that  civilised 
woman  is  nearer  to  the  primitive  type  than  civilised 
man,  for  while  civilised  man  differs  more  from  the 
primitive  type  than  civilised  woman,  so  far  as  absolute 
size  is  concerned,  he  has  made  only  about  half  her 
progress  in  variation,  and  hardly  any  progress  in  corre- 
lation. The  absolute  amount  of  correlation  is  very 
high,  as  the  following  figures  show: 

HALES.  FEMALES. 

ORGANS.  AINO.     FRENCH.      AINO.     FRENCH. 

Femur  and  tibia,  .83  .81  .85  .89 

humerus,  .86  .84  .87  .87 

radius,  .79  .74  .70  .78 

Clavicle  and  humerus,  .44  .63 

Humerus  and  radius,  .78  .85  .74  .85 

Tibia  and  fibula,  .89  .96  .97  .98 

Humerus  and  ulna,  .77  .77  .75  .86 

Radius  and  ulna,  .98  .88  .98  .92 

Here  we  see  that  the  Galtonian  constant,  r,  was  in  most 
instances  above  .8.  Between  the  tibia  and  fibula  it 
averaged  .95,  and  between  radius  and  ulna  .94,  so  that 
in  these  cases  the  correlation  was  almost  absolute. 

In  the  Naquada  race  investigated  by  Warren  *  the 
correlation  between  the  lengths  of  the  long  bones  in 
males  (as  measured  in  about  60  skeletons)  was  distinctly 
higher  than  for  Aino  males,  but  in  females  (measured 
in  about  90  skeletons)  it  was  either  the  same  or  was 
lower  than  in  Aino  females. 

Probably  correlation  is  to  some  extent  affected  by  sex 
in  most  animals.  Thus  Duncker  f  determined  the  cor- 
relation coefficients  of  40  pairs  of  measurements  in  male 

*  Vide  Phil.  Trans.  1898,  B.  p.  178. 

f  Wissenschaf tliche  Meeresuntersuchungen  aus  der  biologischen 
Anstalt  auf  Helgoland,  Bd.  iii.  p.  351. 


84  CORRELATED  VARIATIONS. 

and  female  flounders  (Pleuronectes  flesus),  and  the 
values  obtained  showed  that  the  correlation  was  affected 
by  sex  in  17  out  of  the  40  instances.  The  coefficients 
were  greater  in  the  male  than  in  the  female  fish  in  11 
instances,  and  in  the  female  than  in  the  male  in  6  in- 
stances. Several  of  the  pairs  of  bilateral  homologous 
measurements  (such  as  the  numbers  of  rays  in  the  right 
and  left  pectoral  and  ventral  fins)  showed  distinctly 
lower  correlation  constants  than  were  shown  by  the 
corresponding  pairs  of  measurements  in  the  sym- 
metrical fish  Acerina  cernua  and  Coitus  gdbis.  This 
was  doubtless  due  to  their  possessing  slight  differences 
of  function  in  the  asymmetrical  fish. 

It  should  be  mentioned  that  Pearson,  "Warren,  and 
Duncker  employed  a  somewhat  modified  and  improved 
formula  for  determining  these  correlation  constants,* 
as  compared  with  that  originally  suggested  by  Galton. 
G.  O.  Yule,f  and  also  Pearson  and  Filon,$  have  recently 
shown  that  the  correlation  can  be  determined  in  the 
case  of  skew  variation,  as  well  as  of  normal  variation. 

All  these  results  may  be  taken  to  show  that  every 
part  and  organ  of  the  body  is  correlated  with  every 
other  part  in  a  greater  or  less  degree,  though  such  cor- 
relation may  sometimes  be  of  the  negative  order.  The 
immense  importance  in  evolutionary  processes  of  such 
correlation,  whereby  when  one  organ  becomes  modified 
by  the  action  of  an  agency  such  as  Natural  Selection, 
others  are  modified  also,  is  sufficiently,  obvious  to  need 
no  discussion. 

*Phil.  Trans.,  1896,  A.  p.  264. 
fProc.  Roy.  Soc.,  Ix.  p.  477,  1897. 
{Phil.  Trans.,  1898,  A.  p.  229. 


CORRELATED  VARIATIONS.  85 

Besides  these  instances  in  which  the  degree  of  corre- 
lation can  be  expressed  in  numerical  equivalents,  there 
remain  other  cases  in  which  such  expression  is  difficult 
or  impossible.  In  these  it  must  accordingly  be  de- 
fined in  general  terms.  Darwin  has  collected  a  large 
number  of  such  cases  in  his  "  Animals  and  Plants,"  * 
but  it  is  not  necessary  to  reproduce  more  than  a  few  of 
the  most  striking  of  them  here.  For  instance,  Teget- 
mier  has  stated  that  young  pigeons  of  all  breeds  which 
when  mature  have  white,  yellow,  silver,  blue,  or  dun- 
coloured  plumage,  are  born  almost  naked;  whereas 
pigeons  of  other  colours  are  clothed  with  plenty  of 
down.  Darwin  himself  has  noticed  that  in  feather- 
footed  pigeons,  not  only  does  the  exterior  surface  sup- 
port a  row  of  long  feathers,  like  wing  feathers,  but  the 
very  same  digits  which  in  the  wing  are  completely 
united  by  skin  become  partially  united  by  skin  in  the 
feet.  Again,  Polish  fowls  have  a  large  tuft  of  feathers 
on  their  heads,  and  their  skulls  are  perforated  by 
numerous  holes.  That  this  deficiency  of  bone  is  in  some 
way  connected  with  the  tuft  of  feathers  is  clear  from 
the  fact  of  tufted  ducks  and  geese  likewise  having  per- 
forated skulls.  Constitutional  peculiarities  are  some- 
times correlated  with  colour  in  a  most  curious  and  inter- 
esting manner.  For  instance,  Beddoe  has  shown  that  a 
relation  exists  between  liability  to  consumption  and  the 
colour  of  the  hair,  eyes,  and  skin.  As  regards  ani- 
mals, white  terriers  suffer  most  from  distemper,  white 
chickens  from  a  parasitic  worm  in  their  tracheae,  white 
pigs  from  scorching  in  the  -sun,  and  white  cattle  from 
flies.  Again,  all  the  hogs  in  Virginia,  excepting  those 
*Chap.  xxv. 


86  COEEELATED  VAEIATIONS. 

of  a  black  colour,  suffer  severely  from  eating  the  root  of 
Lachnanthes  tinctoria.  Similarly,  buckwheat  when  in 
flower  is  highly  injurious  to  white  or  white-spotted  pigs, 
if  they  are  exposed  to  the  heat  of  the  sun,  but  quite  in- 
nocuous to  black  pigs. 

These  few  instances  suffice  to  show  how  widespread 
and  apparently  capricious  may  be  the  range  of  correla- 
tion. Until  careful  observations  have  been  made,  ac- 
companied when  possible  by  measurements,  one  can 
never  on  a  priori  grounds  assume  that  there  is  no  cor- 
relation between  particular  organs  or  parts  of  an  organ- 
ism, and  that  an  agency  acting  on  one  part  may  not  at 
the  same  time  be  thereby  indirectly  modifying  another. 

In  their  effects  on  the  modification  of  species,  prob- 
ably by  far  the  most  important  cases  of  correlation  are 
those  in  which  the  reproductive  system  is  concerned. 
Until  recently  comparatively  little  attention  was  paid 
to  such  phenomena,  and  probably,  even  now,  they  are 
far  from  being  estimated  at  their  true  value  by  many 
biologists.  Perhaps  the  reason  of  this  lies  in  the  fact 
that  the  physiological  condition  of  an  organism,  or  its 
relative  degree  of  sexual  compatibility  with  other  or- 
ganisms of  its  own  and  of  different  species,  is  so 
exceedingly  difficult,  if  not  impossible,  to  estimate. 
Thus  the  degree  of  fertility  cannot  be  tested  in  more 
than  one  or  two  instances  with  each  individual  or- 
ganism— at  least  in  the  higher  animals — and  so  the 
desired  information  can  only  be  acquired  by  carry- 
ing out  most  lengthy  and  laborious  series  of  observa- 
tions. There  are  sufficient  data  at  our  disposal,  how- 
ever, to  indicate  that  the  reproductive  system  is  no 
less  subject  to  variation  than  any  other  part  of  the 


CORRELATED  VARIATIONS.  8? 

organism,  and  indeed  is  probably  a  good  deal  more  so. 
Supposing  that  the  quality  of  fertility  is  correlated 
with  some  particular  character  or  characters  more  than 
it  is  with  other  characters,  then  it  follows  that  more  in- 
dividuals bearing  the  character  in  question  will  be  born 
and  propagate  their  kind,  and  so,  in  course  of  time,  the 
whole  race  will  be  modified  in  this  direction.  This  prin- 
ciple has  been  termed  by  its  discoverer,  Professor  Pear- 
son, "  Reproductive  "  or  "  Genetic  Selection."  Its 
existence  as  a  real  factor  in  evolution  depends  on  the 
validity  of  the  assumption  that  the  characteristic  of 
fertility  is  inherited.  That  this  is  so,  Professor  Pear- 
son, in  conjunction  with  Miss  Alice  Lee  and  Mr.  L. 
Bramley-Moore,*  has  recently  proved  by  a  most  labori- 
ous research  on  inheritance  in  man  and  in  the  thor- 
oughbred race-horse.  Their  results  show  that  fertility 
is  undoubtedly  inherited  from  mother  to  daughter,  and 
also  from  father  to  son.  It  was  also  found  that  a 
woman's  fertility  is  as  highly  correlated  with  that  of 
her  paternal  as  with  that  of  her  maternal  grandmother. 
In  other  words,  the  latent  character  fertility  in  the 
woman  is  transmitted  through  the  male  line,  and  with 
an  intensity  which  approximates  to  that  required  by  the 
law  of  ancestral  heredity.  Again,  it  was  deduced  that 
fecundity  in  the  brood-mare  is  inherited  from  dam  to 
mare,  and  also  from  grand-dam  to  mare  through  the 
dam.  Also  the  latent  quality  of  fecundity  in  the  brood- 
mare is  inherited  through  the  sire,  and  by  the  stallion 
from  his  sire.  In  these  latter  two  cases,  the  degree  of 
inheritance  approaches  fairly  closely  to  that  required  by 
Galton's  law  of  ancestral  heredity,  but,  in  the  two  for- 
*Proc.  Roy.  Soc.,lxiv.  p.  163,  1899. 


88  CORRELATED  VARIATIONS. 

mer,  it  is  much  less.  As,  therefore,  fertility  is  proved 
to  be  inherited  in  man,  and  fecundity  in  the  horse,  it  is 
probable  that  both  these  characters  are  inherited  in  all 
classes  of  life. 

The  importance  of  this  theory  of  Genetic  Selection 
will,  perhaps,  be  better  realised  by  quoting  a  concrete 
case  concerning  man,  this  being  the  only  one  in  which 
statistics  are  at  present  available.  Working  on  data  con- 
cerning 4000  families,  principally  of  the  Anglo-Saxon 
race,  and  1842  families  of  the  Danish  race,  Professor 
Pearson  *  determined  that  there  is  a  sensible  correla- 
tion (about  .18)  between  fertility  and  height  in  mothers 
of  daughters.  Supposing  genetic  selection  to  have  been 
unchecked  by  natural  selection,  say  for  forty  genera- 
tions, the  mean  height  of  women  would  have  been  raised 
about  3J  inches.  A  factor  which  would  alter  stature 
by  about  three  inches  in  1000  years  is  clearly  capable 
of  producing  very  considerable  results  in  the  long 
periods  during  which  evolution  may  be  supposed  to  have 
been  at  work.  The  importance  of  the  influence  of 
genetic  selection  in  the  case  of  man  is  also  shown  by  the 
fact  that,  as  proved  by  these  statistics,  less  than  a  quar- 
ter of  one  generation,  by  reason  of  their  fertility,  pro- 
duce more  than  half  of  the  next  generation.  Correla- 
tion between  fertility  and  any  mental  or  physical  char- 
acteristic must  therefore  work  a  progressive  change. 
For  example,  arguing  from  the  class  fertility  statistics 
which  have  been  determined  among  the  population  of 
Copenhagen,  it  is  gathered  that  the  artisan  class  pro- 
duce a  larger  proportion  of  children  than  the  profes- 
sional classes,  as  their  gross  fertility  is  greater,  and 
*Proc.  Roy.  Soc.,  lix.  p.  301,  1896. 


CORRELATED  VARIATIONS.  83 

their  marriage  rate  is  so  much  higher.  This  increased 
fertility  is  somewhat  counteracted  by  their  greater 
death  rate,  but  it  would  nevertheless  appear  that  the 
population  will  ultimately  reproduce  itself  from  the 
artisan  classes. 

Definite  evidence  of  evolution  under  natural  condi- 
tions as  the  result  of  genetic  selection  has  not  been  ob- 
tained, but  this  may  be  simply  because  it  has  never 
occurred  to  anybody  to  look  for  it.  Professor  Pear- 
son *  has,  however,  shown  the  existence  of  a  highly 
interesting  and  important  relationship  between  fer- 
tility and  morphological  characters  in  certain  plants. 
He  counted  the  number  of  stigmatic  bands  on  the  4443 
seed-capsules  obtained  from  176  Shirley  poppies  grow- 
ing in  a  single  garden,  and  found  the  following  fre- 
quencies : 

Bands,  5678      9    10    11    12    13    14    15    16  17  18  19 

Frequency,    1  11  32  56  148  363  628  925  954  709  397  155  51  12    1 

Here  we  see  that  the  12  and  13  band  forms  were  the 
most  common,  the  11  and  14  ones  less  so,  the  10  and  15 
still  less,  and  so  on.  To  his  surprise^  Professor  Pear- 
son found  that  whilst  the  commonest  or  type  capsules 
contained  a  very  large  number  of  seeds,  the  11  and  14 
band  forms  contained  distinctly  less,  and  the  10  and  15 
ones  very  few  seeds  indeed,  whilst  the  capsules  with 
very  few  or  very  many  bands  contained  practically  no 
seeds.  A  repetition  of  these  observations  on  the  wild 
poppy  gave  a  very  similar  result,  and  this  was  likewise 
the  case  with  the  seed  capsules  of"  a  number  of  plants 
of  Nigella  Hispanica.  The  distribution  of  the  seg- 
mentation on  3212  capsules  was  as  follows: 
*  "  Grammar  of  Science,"  Ed.  ii.  p.  443. 


90  CORRELATED  VARIATIONS. 

No.  of  Segments,  234  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20 
Frequency,  10  7  20  303  412  534  1552  223  59  35  43  6  6  2 

In  this  plant  Pearson  found  the  8  segment  capsules 
to  be  highly  fertile,  whilst  in  the  10,  11,  and  12  segment 
capsules  he  could  find  hardly  any  seed  at  all.  Six  and 
7  segment  capsules  were  only  moderately  fertile,  and 
those  with  5  segments  or  less  were  practically  sterile. 
These  experiments  Professor  Pearson  holds  to  illustrate 
a  very  important  law,  namely,  "  Fertility  is  not  uni- 
formly distributed  among  all  individuals,  but  for  stable 
races  there  is  a  strong  tendency  for  the  character  of 
maximum  fertility  to  become  one  with  the  character 
which  is  the  type."  It  follows,  therefore,  if  this  prin- 
ciple is  generally  true,  that  stable  races  are  very  largely 
the  product  of  the  typical  or  most  frequently  occurring 
members,  and  not  of  all  the  individual  members  in  pro- 
portion to  their  numbers.  There  would  seem  to  be  a 
constant  tendency  to  keep  the  type  uniform  and  limit 
the  variability  in  either  direction  as  much  as  possible. 
Doubtless  under  changed  conditions  of  environment, 
the  relative  fertility  might  also  become  changed,  and  in 
consequence  a  gradual  evolution  result. 

A  similar  relationship  between  fertility  and  type 
form  has  been  noticed  by  Davenport  *  in  one  of  the 
Hydromedusse,  Pseudoclytia  pentata.  This  organism 
differs  from  all  other  Hydromedusse  in  that  it  normally 
has  five  radial  canals,  instead  of  four.  A.  G.  Mayer  t 
has  examined  the  variation  of  the  species,  and  obtained 
the  frequencies  given  in  the  subjoined  table.  From 
these  values  we  gather  that  the  four  and  six  canal  forms 

*Biometrika,  i.  p.  255,  1902. 

\  Science  Bulletin  of  the  Brooklyn  Museum,  vol.  i. 


COERELATED  VARIATIONS.  91 

are  comparatively  common,  whilst  other  abnormalities 
are  rare.  Typically  one  gonad  (or  reproductive  organ) 
occurs  on  each  radial  canal,  but  on  an  average  about 
one  in  seven  of  them  fails  to  develop.  In  atypical 
forms,  however,  as  can  be  gathered  from  the  table,  the 
proportion  failing  to  develop  is  found  to  be  larger  and 
larger,  the  further  the  departure  of  the  individual  from 
the  type.  In  such  forms  as  depart  from  the  normal 
radial  symmetry,  even  if  they  still  possess  five  rays,  the 
partial  sterility  is  very  much  increased. 

PER  CENT.  OF  GONADS  FAILING  TO  DEVELOP. 

NUMBER  OP         FREQUENCY  OF  IRREGULAR 

CANALS.  OCCURRENCE.  ALL  INDIVIDUALS.  INDIVIDUALS. 

2  1 

3  8  25.0  44.5 

4  56  21.8  31.0 

5  860  15.6  37.0 

6  64  18.6  31.0 

7  6  21.5  43.0 

8  1 

A  very  interesting  case  of  variation  described  by  Bate- 
son  *  may  perhaps  be  ascribed  to  the  action  of  genetic 
selection,  though  there  is  no  direct  evidence  to  warrant 
the  assumption.  It  concerns  the  Peppered  moth, 
Amphidasys  betularia.  A  striking  black  variety  of  this 
insect,  A.  doubledayaria,  was  first  met  with  as  a  rarity 
in  1840-50.  Since  then  its  numbers  have  gradually  in- 
creased, till  in  1870  about  equal  numbers  of  the  pure 
type  and  of  its  variety  occurred  at  Monmouth,  whilst  a 
few  years  later  the  typical  form  had  entirely  disap- 
peared. At  Chester  none  but  black  forms  have  been 
met  with  for  many  years.  In  the  south  of  England, 
however,  the  typical  form  is  still  alone  present.  Bate- 
*  Science  Progress,  vol.  vi.  p.  557,  1897. 


92  CORRELATED  VARIATIONS. 

son  suggests  that  this  gradual  replacement  of  a  type  by 
its  variety  is  probably  due  to  success  in  the  struggle  for 
existence  of  this  particular  dark  strain,  but  it  may 
equally  well  be  accounted  for  by  supposing  that  it  is  the 
result  of  a  greater  fertility.  Intermediate  strains  are 
not  unknown,  for  in  Belgium  it  seems  clear  that  one 
has  succeeded  in  establishing  itself,  and  in  England  it 
is  probable  that  they  were  once  more  common  than  they 
are  now.  Intermediate  forms  are  said  to  be  plentiful 
also  in  the  Rhenish  Provinces  and  Westphalia,  and  the 
same  is  true  of  the  black  forms.  Bateson  says  there  is 
no  doubt  that  the  black  variety  existed  at  an  early  stage 
in  the  transformation,  side  by  side  with  the  light  one. 
The  course  of  events  has  not  been  that  the  insects  of 
each  successive  district  have  become  more  and  more 
tinged  with  black,  till  they  culminated  in  A.  double- 
dayaria,  but  rather  that  this  variety,  or  less  often  one 
of  the  intermediate  forms,  spread  into  or  at  least  ap- 
peared in  the  area,  and  either  coexists  with  the  type  or 
has  replaced  it. 

The  few  breeding  experiments  thus  far  made  on  the 
moth  show  that  there  is  an  imperfect  blending  of  type 
and  variety.  Steinert  raised  from  a  black  wild  female 
a  brood  of  75  typical  and  90  varietal  forms,  but  there 
were  no  really  intermediate  ones,  though  two  of  the 
examples  classed  as  betularia  were  darker  than  the  nor- 
mal. That  this  female  had  paired  with  a  typical  male 
— the  progeny  thus  resembling  either  the  one  parent  or 
the  other,  but  not  both — is  shown  by  the  following  exj 
periment,  also  recorded  by  Bateson.*  W.  H.  B. 
Fletcher  tied  out  a  black  female,  which  had  been  reared 
*  Science  Progress,  vol.  vii.  p.  53,  1898. 


CORRELATED  VARIATIONS.  93 

in  captivity.  This  was  at  Worthing,  where  the  typical 
male  form  is  the  only  one  known,  so  that  the  brood  ob- 
tained were  most  certainly  produced  from  a  cross  of  the 
two  varieties.  The  offspring  were  sharply  divided  into 
10  male  and  8  female  betularidj  and  6  male  and  5  fe- 
male doubledayaria. 

Conversely  to  the  modification  of  a  community  in 
one  direction  by  reason  of  the  increased  productiveness 
of  certain  of  its  members,  we  may  experience  modifica- 
tion in  the  opposite  direction  by  reason  of  a  decreased 
productiveness.  For  instance,  Dr.  Beddoe  *  states  that 
there  is  a  good  deal  of  evidence  as  to  the  greater 
liability  of  blonds  than  of  brunets  to  certain  classes 
of  disease.  At  least  this  is  so  in  America,  as  has  been 
shown  by  Baxter,  f  Thus  it  would  appear  that  the 
blonds  have  less  chance  than  the  brunets  of  contribut- 
ing their  due  proportion  to  the  next  generation,  and  so 
they  must  be  relatively  diminishing  in  numbers.  That 
this  is  the  case  is  supported  by  the  fact  that  of  Ameri- 
cans accepted  for  service  in  the  army  a  greater  propor- 
tion were  brunets  than  of  the  English,  Irish,  and 
Germans  accepted.  Thus: 

Among  the  Americans  were  66  light  and  34  dark  complexioned 

"     English          "      70    "  "    30     " 

"    Irish  "      70    "  "    30     '• 

"    Germans        "      69    "  "    29     " 

The  fact  that  most  species  are  to  some  extent  mu- 
tually sterile,  whilst  their  hybrid  offspring  are  almost 
invariably  so,  proves  that  the  physiological  condition  of 

*  Science  Progress,  vol.  v.  p.  384,  1896. 

f  "Medical  Statistics  of  the  Provost-Marshal-General's  Bureau," 
Washington,  1875. 


94  CORRELATED  VARIATIONS. 

the  reproductive  system  is  closely  correlated  with  the 
physiological  condition  of  the  organism  taken  as  a 
whole.  Recent  physiological  research  has  taught  us 
that  most  organs  in  the  body,  in  addition  to  their  more 
obvious  functions,  have  an  internal  secretion  which 
passes  into  the  circulation  of  the  body,  and  there  exerts 
some  important,  but  unexplained,  influence  on  the  gen- 
eral metabolism  of  the  tissues.  Deprivation  of  such 
internal  secretion,  by  extirpation  of  one  of  these  organs, 
may  speedily  upset  the  normal  working  of  many  or 
most  of  the  other  tissues  of  the  body,  and  ultimately  re- 
sult in  death.  Every  organ  and  tissue  of  the  body 
probably  reacts  on  every  other  organ,  and  modifies  its 
physiological  condition,  and  thereby  may  ultimately 
produce  structural  changes.  The  reproductive  system 
is  apparently  more  sensitive  than  most  other  organs, 
and  hence  is  very  readily  affected  by  any  changes  in  the 
condition  of  the  organism  as  a  whole.  For  instance,  it 
is  well  known  that  most  animals  refuse  to  breed  in  con- 
finement, though  they  can  be  kept  for  many  years  in  a 
condition  of  perfect  health.  The  changed  conditions 
of  life  must  therefore  have  acted  on  the  organism  as  a 
whole,  so  as  to  modify  certain  of  its  internal  secretions, 
and  these,  reacting  on  the  reproductive  system,  have 
brought  about  the  observed  sterility.  If  a  considerable 
change  in  conditions  of  life  produces  complete  sterility, 
it  seems  highly  probable  that  slight  changes  in  such 
conditions  may  produce  a  partial  sterility,  or  a  differ- 
ential fertility.  Thus  organisms  in  a  state  of  nature, 
if  exposed  to  a  change  of  climate,  the  result  of  migra- 
tion or  stress  of  weather,  or  to  a  change  in  their  food, 
may  have  the  physiological  condition  of  their  repro- 


CORRELATED  VARIATIONS.  95 

ductive  system  slightly  altered,  whereby  the  principles 
of  Genetic  Selection  and — in  the  case  of  plants — of 
Physiological  Selection,  may  become  effective,  and 
modify  or  split  up  the  species. 

Just  as  the  condition  of  the  organism  as  a  whole  may 
modify  that  of  the  reproductive  system,  so  may  the  con- 
dition of  the  reproductive  system  modify  that  of  the 
organism.  The  difference  between  the  spirit  and  ap- 
pearance of  castrated  animals  and  th#t  of  normal  ani- 
mals is  sufficiently  well  known  to  need  no  remark. 
Such  differences  must  be  due  in  large  part  to  the  lack  of 
internal  secretion  from  the  organs  of  reproduction. 
Striking  as  is  the  influence  of  the  reproductive  organs 
on  the  physiological  condition  of  an  animal,  that  upon 
the  morphological  structure  is  even  more  noteworthy. 
That  castrated  male  animals  fail  to  develop  their  sec- 
ondary sexual  characteristics,  is  notorious.  Thus,  if  the 
operation  be  performed  upon  a  young  cock,  he  never 
crows  again;  the  comb,  wattles,  and  spurs  do  not  grow 
to  their  full  size,  and  the  hackles  assume  an  inter- 
mediate appearance  between  true  hackles  and  the 
feathers  of  the  hen.  Conversely,  it  is  well  known  that 
a  large  number  of  female  birds,  such  as  fowls,  various 
pheasants,  partridges,  pea-hens,  and  ducks,  when  old  or 
diseased,  or  when  operated  upon,  assume  many  or  all  of 
the  secondary  male  characters  of  their  species.  Water- 
ton  gives  a  curious  case  of  a  hen  which  had  ceased  lay- 
ing, and  had  assumed  the  plumage,  voice,  spurs,  and 
warlike  disposition  of  the  cock.  Again,  the  females  of 
two  kinds  of  deer,  when  old,  have  been  known  to  ac- 
quire horns.* 

"Animals  and  Plants,"  vol.  ii.  p.  26. 


96  CORRELATED  VARIATIONS. 

Analogous  or  Parallel  Variation.  This  term  has 
been  used  by  Darwin  to  indicate  that  similar  characters 
occasionally  make  their  appearance  in  several  varieties 
or  races  descended  from  the  same  species,  and  more 
rarely  in  the  offspring  of  widely  distinct  species.*  It 
is  unnecessary  to  make  more  than  very  brief  mention  of 
this  subject,  because,  as  Darwin  points  out,  the  majority 
of  observed  cases — such  as  the  occasional  appearance  of 
black  wing  bars  in  the  various  breeds  of  pigeon,  and  of 
stripes  on  the  legs  of  the  ass  and  of  various  races  of  the 
horse — are  evidently  due  to  reversion.  The  others  are 
probably  mere  coincidences,  and  of  no  scientific  value. 
Among  these  latter,  Darwin  mentions  the  fact  that 
many  trees  belonging  to  quite  different  orders  have  pro- 
duced pendulous  and  f astigate  varieties.  A  multitude 
of  plants  have  yielded  varieties  with  deeply  cut  leaves. 
Several  varieties  of  melon  resemble  other  species  in  im- 
portant characters.  Thus  one  variety  has  fruit  so  like, 
both  externally  and  internally,  the  fruit  of  the  cucum- 
ber, as  hardly  to  be  distinguished  from  it.  In  animals, 
again,  we  find  feather-footed  races  of  the  fowl,  pigeon, 
and  canary  bird.  Horses  of  the  most  different  races 
present  the  same  range  of  colour.  Many  sub-varieties 
of  the  pigeon  have  reversed  and  somewhat  lengthened 
feathers  on  the  back  parts  of  the  head. 

In  connection  with  this  subject  of  parallel  variation, 
Walsh's  "  Law  of  Equable  Variability  "  f  may  be  men- 
tioned. This  states  that  "  if  any  given  character  is 
very  variable  in  one  species  of  a  group,  it  will  tend  to 
be  variable  in  allied  species ;  and  if  any  given  character 

*  "  Animals  and  Plants,"  vol.  ii.  p.  340. 

f  Proc.  Entomolog.  Soc.  Philadelphia,  p.  213,  1863. 


CORRELATED  VARIATIONS.  97 

is  perfectly  constant  in  one  species  of  a  group,  it  will 
tend  to  be  constant  in  allied  species."  The  general 
truth  of  this  law  seems  highly  probable  on  the  face  of 
it,  because  most  allied  species  have  presumably  split  off 
from  their  common  ancestor  at  no  very  remote  period, 
and  so  would  still  resemble  each  other  more  or  less 
closely  in  respect  of  variability,  just  as  they  do  in  re- 
spect of  morphological  structure.  The  results,  already 
quoted,  of  Weldon  and  Warren  for  the  correlation  of 
various  organs  in  Carcinus  and  Portunus,  afford  direct 
experimental  evidence  in  support  of  this  law. 

In  conclusion,  it  may  not  be  out  of  place  to  make  one 
or  two  brief  remarks  as  to  the  general  bearing  of  the 
evidence  which  has  been  adduced  in  these  three  chap- 
ters concerning  the  "  facts  of  variation."  I  believe 
that  they  include  most  of  the  more  important  and  more 
recent  contributions  to  our  knowledge  of  the  subject, 
especially  those  in  which  the  results  have  been  ex- 
pressed in  exact  numerical  terms.  It  may  very  likely 
be  objected  that  insufficient  importance  has  been  at- 
tached to  the  kind  of  information  which  Darwin  col- 
lected so  thoroughly  and  in  such  profusion  in  his  work 
on  "  Variation  in  Animals  and  Plants  under  Domestica- 
tion." 

The  reason  of  this  is  twofold.  In  the  first  place 
it  seemed  unnecessary  to  refer  at  great  length  and 
with  much  frequency  to  data  with  which  most  seri- 
ous students  of  Biology  must  be  already  conversant: 
whilst  in  the  second  place,  comparatively  little  infor- 
mation of  this  kind  has,  as  far  as  I  am  aware,  been 
recorded  in  scientific  journals  since  Darwin's  time.  It 
has,  indeed,  been  recognised  that  the  facts  of  variation 


98  CORRELATED  VARIATIONS. 

attain  a  much  higher  and  more  permanent  value  in  pro- 
portion as  they  are  expressed  in  exact  numerical 
terms. 

To  say  that  some  organism  or  part  of  an  organism  is 
more  variable  than  another  is  very  unsatisfactory,  com- 
pared with  the  statement  that  the  variabilities  of  cer- 
tain characters  in  the  one  are  of  such  and  such  values, 
and  in  the  other,  of  certain  other  values.  From  such 
data  as  these  we  can  compare  the  variabilities  of  all  the 
variants  exactly,  both  with  each  other  and  with  any 
other  variants,  and  determine  what  relation,  if  any, 
they  bear  to  their  systematic  importance.  We  can  tell 
if  the  variations  obey  the  normal  law  of  error,  or  if  they 
are  asymmetrical  in  their  distribution.  In  this  latter 
case,  we  may  be  able  to  discover  the  existence  of  a 
tendency  to  divergence  or  splitting  up  of  a  species,  in 
its  very  earliest  stages.  Repetition  of  our  observa- 
tions at  some  future  period  would  thus  become  a  sub- 
ject of  especial  interest,  as  we  might  in  such  a  case  hope 
to  detect  some  change  both  in  the  mean  values  and  in 
the  distribution  of  the  variations,  indicating  that  the 
evolutionary  process  was  still  progressing,  and  the  di- 
rection in  which  the  progress  was  being  made.  Fur- 
ther, as  we  shall  see  in  a  subsequent  chapter,  by  deter- 
mining the  average  characters  of  groups  of  individuals 
which  have  been  subjected  for  some  period  to  the 
struggle  for  existence,  and  comparing  them  with  the 
characters  of  other  individuals  which  have  not  been  ex- 
posed to  such  a  struggle,  or  by  comparing  the  characters 
of  individuals,  which  owing  to  the  severity  of  the  strug- 
gle for  existence  had  been  actually  eliminated,  with  the 
characters  of  the  survivors,  we  are  able  to  obtain 


CORRELATED  VARIATIONS.  99 

numerical  proof  of  the  action  of  Natural  Selection. 
Still  again,  by  comparing  the  characters  of  individuals 
with  those  of  their  parents  and  of  their  offspring,  we 
are  enabled  to  work  out  with  exactness  the  origin  and 
transmission  of  such  characters,  and  so  elucidate  the 
Laws  of  Heredity. 


PART  II. 
THE   CAUSES   OF  VAKIATIOK 

CHAPTEE  IV. 

BLASTOGENIC     VARIATIONS. 

The  ultimate  cause  of  blastogenic  variation—Effect  of  staleness  and 
of  comparative  maturity  of  sex-cells  on  the  characters  of  organisms 
— Amphimixis— Identical  twins — Transplantation  of  ova  in  the 
rabbit — Law  of  Ancestral  Heredity  in  man  and  in  the  Basset 
hound— Regression  towards  mediocrity— Exclusive  inheritance.— 
Homotyposis. 

AKGUING  from  his  theory  of  the  continuity  of  the 
germ-plasm,  first  suggested  in  1883,*  Weismann  came 
to  the  conclusion  that  acquired  characters  were  not 
transmissible.  Such  acquired  characters  are  due  to  the 
direct  influence  of  the  environment  upon  the  body 
tissues  of  an  organism,  or  are  variations  of  somatogenic 
origin.  Opposed  to  these  are  variations  due  directly  to 
certain  peculiarities  of  the  germ-plasm2  or  variations  of 
blastoger^ic  origin,  which  are,  on  the  contrary,  hereditary 
or  transmissible.  Thus,  according  to  Weismann,  all 
variations  are,  in  respect  of  their  origin,  sharply  divisi- 
ble into  these  two  groups,  whilst  in  respect  of  trans- 
missibility  they  are  equally  distinct.  In  the  present 
*Vide  "  Essays  on  Heredity,"  Oxford,  1889,  p.  71. 

101 


102  BLASTOGENIC  VARIATIONS. 

chapter  these  blastogeriic,  genetic,  or  germinal  varia- 
tions will  be  discussed,  whilst  somatogenic  variations 
will  be  treated  of  later. 

The  cause  of  hereditary  variation  Weismann  ascribes 
to  the  direct  effect  of  external  influences  on  the  so- 
called  biophors  and  determinants  of  the  germ-plasm.* 
These  biophors  Weismann  defines  as  "  bearers  of 
vitality/7  or  the  smallest  units  of  protoplasm  which 
exhibit  the  primary  forces  of  assimilation^anct  metab- 
olism, growth,  and  multiplication  by  fission.  All  pro- 
toplasm, both  the  nucleus  and  body  of  cells,  is  made  up 
of  these  units.  They  are  the  bearers  of  the  qualities  or 
characters  of  the  cells.  Determinants,  on  the  other 
hand,  are  groups  of  biophors,  and  are  the  particles  of 
germ-plasm  corresponding  to  and  determining  the  cells 
or  groups  of  cells  which  are  independently  variable 
from  the  germ  onwards. 

The  reaction  of  the  germ-plasm  to  external  influ- 
ences is  primarily  one  of  nutrition.  The  biophors  and 
determinants  are  supposed  to  be  subject  to  continual 
changes  of  composition  during  their  almost  uninter- 
rupted growth,  and  these  very  minute  fluctuations  are 
the  primary  cause  of  the  greater  deviations  of  the  de- 
terminants, which  are  finally  observed  in  the  form  of 
individual  variations.  The  growing  determinants  must 
originally  differ  to  some  slight  extent  in  the  composi- 
tion of  their  biophors,  as  otherwise  inequalities  of  nutri- 
tion could  never  effect  any  transformation,  but  could 
only  alter  their  rate  of  growth.  Slight  as  are  these 
deviations  in  the  determinants  effected  by  inequalities 
of  nutrition,  they  are  nevertheless  of  great  significance, 
*  ••  The  Germ-Plasm,"  London,  1893,  p.  415. 


BLASTOGENIC  VARIATIONS.  103 

as,  by  a  process  of  accumulation,  they  form  the  material 
from  which  the  visible  individual  variations  are  pro- 
duced by  means  of  "  amphimixis." 

Amphimixis  is  that  form  of  reproduction  which  is 
found  in  all  the  higher  organisms,  and  which  consists  in 
the  mingling  of  two  individuals  or  their  germs;  i.  e., 
the  so-called  sexual  reproduction.  The  term  is  also  ap- 
plied to  a  similar  phenomenon  occurring  amongst  uni- 
cellular organisms,  i.  e.,  to  conjugation.  In  this  case 
reproduction  is  not  a  necessary  or  even  usual  concomi- 
tant, but  takes  place  independently.  To  amphimixis 
Weismann  attributes  the  constant  occurrence  of  indi- 
vidual variability,  although  he  recognises  that  it  is  not 
the  primary  cause  of  this  variability;  but  rather  the 
process  furnishes  an  inexhaustible  supply  of  fresh  com- 
binations of  individual  variations.  Thus  the  germ- 
plasm  of  a  new  individual  produced  by  amphimixis 
never  receives  more  than  half  the  ids  of  each  parent, 
and  these  are  differently  selected  and  arranged  in  each 
case.  By  an  id,  it  may  be  remarked,  "Weismann  means 
a  group  of  determinants  which  contains  all  the  deter- 
mining elements  of  the  species,  though  in  a  manner 
peculiar  to  the  individual. 

Blastogenic  variations  are  thus,  according  to  Weis- 
mann, primarily  dependent  on  two  chief  factors:  (1) 
Inequalities  of  nutrition  acting  on  the  individual  con- 
stituents of  the  germ-plasm;  (2)  Amphimixis. 

It  behoves  us  to  examine  these  two  factors  more 
closely,  and  see  how  far  they  are  supported  by  experi- 
mental evidence.  It  is,  on  the  face  of  it,  impossible  to 
put  Weismann's  hypothesis  of  the  reaction  of  determi- 
nants to  inequalities  of  nutrition  to  a  practical  test,  but 


104  BLASTOGENIC  VAEIATIONS. 

we  can  at  least  enquire  into  what  is  known  about  the  in- 
fluence of  nutrition  on  the  germ-plasm  as  a  whole.  In 
fact,  we  can  see  how  far  the  parental  plasms  are  indi- 
vidually capable  of  being  affected  by  changes  of  nutri- 
tion, so  as,  on  subsequent  mingling  in  sexual  union,  to 
give  rise  to  appreciable  changes  in  the  resulting  off- 
spring. Important  as  this  subject  is,  the  direct  experi- 
mental evidence  available  upon  it  is  distinctly  meagre. 
It  is,  for  instance,  probable  that  the  children  of  a  father 
whose  tissues,  and  therefore  his  sex-cells,  are  saturated 
with  alcohol  or  the  products  of  some  disease,  are 
smaller  and  less  well  formed  than  those  of  normal 
parents,  but  there  are  no  satisfactory  data  to  support  it. 
Similar  evidence  with  reference  to  the  female  sex- 
cells  is  obviously  not  available,  as  any  effects  produced 
on  offspring  would  probably  in  chief  part  arise  during 
embryonic  development,  or  after,  and  not  before,  fer- 
tilisation. 

The  evidence  obtained  as  to  the  influence  of  nutri- 
tion on  the  evolution  of  sex  is  only  indirectly  related  to 
the  problem  under  discussion.  Most  of  it  goes  to  show 
that  increased  feeding  of  young  organisms  tends  to- 
wards the  production  of  a  larger  proportion  of 
females,*  and  hence,  as  male  and  female  sex-cells 
cannot  be  considered  entirely  equivalent,  it  follows 
that  an  effect  is  produced  on  the  germ-plasm.  Yet 
there  is  no  evidence  to  show  that  the  offspring  of  fe- 
males which  arose  in  spite  of  bad  feeding  differ  in  any 
way  from  those  of  females  produced  in  consequence  of 
good  feeding.  Nevertheless  it  seems  probable  that,  as 
nutrition  has  some  influence  in  determining  the  sexual 
*  Vide  Geddes  and  Thomson's  "  Evolution  of  Sex,"  p.  41,  1889. 


BLASTOGENIC  VARIATIONS.  105 

character  of  the  germ-plasm  in  a  developing  animal 
after  fertilisation,  it  may  also  have  some  influence  if  it 
be  brought  to  bear  on  the  parental  germ-plasms  before 
fertilisation.  It  seems  likely,  in  fact,  that  a  highly 
nourished  ovum,  as  compared  with  one  less  favourably 
conditioned,  will  tend  rather  to  a  female  than  a  male 
development. 

Evidence  bearing  more  directly  on  the  question  at 
issue  has  recently  been  obtained  by  the  author,  in  a 
research  on  the  effect  of  staleness  of  the  sex-cells  on 
development.*  The  method  of  experiment  was  to  keep 
the  ova  or  spermatozoa,  or  both,  of  the  sea-urchin 
Strongylocentrotus  lividus  for  varying  numbers  of 
hours  in  sea  water  before  permitting  fertilisation,  and 
after  eight  days'  development  to  measure  the  length  of 
the  larvae  and  see  if  they  differed  in  size  from  normal 
larvae.  As  other  observations  on  larvae  obtained  from 
these  artificial  fertilisations  will  be  referred  to  later, 
the  experimental  procedure  adopted  may  be  briefly  in- 
dicated. It  consisted  in  shaking  pieces  of  the  ovaries 
and  testes  of  several  ripe  specimens  of  the  Echinoid 
in  small  jars  of  water,  and  mixing  portions  of  their  con- 
tents either  immediately,  or  after  a  given  number  of 
hours.  The  mixed  solutions  were  allowed  to  stand  for 
an  hour,  and  were  then  poured  into  large  jars  holding 
from  2  to  4  litres  of  sea-water.  These  were  placed  in 
a  tank  of  running  water,  whereby  the  temperature  was 
kept  nearly  constant,  it  varying  less  than  a  degree  dur- 
ing twenty-four  hours  and  not  more  than  two  degrees 
during  the  whole  course  of  the  experiment.  The  fer- 
tilised ova  were  allowed  to  develop  for  eight  days,  as  by 
*Proc.  Roy.  Soc.,  vol.  Ixv.  p.  350,  1899. 


106 


BLASTOGENIC  VAEIATIONS. 


that  time  the  arms  of  the  larvae  or  plutei  have  attained 
their  maximum  length,  whilst  the  body  has  practically 
ceased  growing.  The  larvae  were  then  killed  by  adding 
corrosive  sublimate.  They  were  collected  and  pre- 
served in  80  per  cent,  alcohol,  and  were  subsequently 
mounted  in  glycerine  and  measured  under  the  micro- 


FIGS.  17  AND  18. — Larvae  of  Strongylocentrotus  limdus. 

scope  by  means  of  a  micrometer  eyepiece.  The  body- 
length,  AB,  was  always  measured  in  50  different  larvae, 
and  a  mean  taken.  In  many  cases  also  the  anal  arm- 
length,  AC,  and  sometimes  also  the  oral  arm-length,  AD 
were  measured  as  well,  and  these  measurements  calcu- 
lated as  percentages  on  the  body  length. 

In  order  to  determine  the  effect  of  staleness  of  the 
sex-cells  on  the  size  of  the  larvae,  five  series  of  measure- 
ments had  to  be  made,  viz.,  (1)  of  the  normal  larvae  ob- 
tained from  the  fresh  ova  fertilised  with  the  fresh 


BLASTOGENIC  VARIATIONS. 


107 


sperm,  (2)  those  from  stale  ova  and  stale  sperm,  (3) 
from  stale  ova  and  fresh  sperm  obtained  from  another 
freshly  opened  Echinoid,  (4)  from  fresh  ova  and  stale 
sperm,  (5)  and  lastly,  from  the  ova  and  sperm  of  the 
freshly  opened  Echinoids.  It  was,  of  course,  impos- 
sible to  get  an  exact  basis  of  comparison  for  the  larvae 
obtained  from  one  stale  and  one  fresh  sexual  product. 
The  best  possible  was  to  take  a  mean  between  the  size 
of  the  original  normal  larvae,  and  that  of  the  larvae  ob- 
tained from  the  fresh  sexual  products  used  f o*  fertilis- 
ing the  stale  products.  The  larvae  obtained  with  both 
sexual  products  stale  are,  of  course,  accurately  com- 
parable with  the  original  normal  larvae.  In  the  ac- 
companying table  an  example  is  given  of  the  mean  per- 
centage differences  in  the  size  of  the  larvae  obtained 
with  fresh  and  stale  products,  from  the  original  nor- 
mal larvae  in  the  one  case,  and  from  the  mean  between 
the  original  and  fresh  normal  larvae  in  the  other  two 


FERTILIZATION  MADE  AFTER 

CONDITION  OF 

SEXUAL  CELLS. 

9  HRS. 

24  HRS. 

33  HRS. 

45  HRS. 

Stale   9,  stale  $ 

-0.2 

+1.9 

+  1.1 

-  1.9 

Fresh  9,  stale  $ 

+7.1 

+3.7 

+10.9 

+  1.5 

Stale  $  ,  fresh  $ 

-2.8 

-3.0 

+  2.0 

-15.9 

cases.  In  this  experiment,  which  was  the  most  com- 
plete made,  the  fertilisations  were  performed  after 
keeping  the  sexual  products  for  respectively  9,  24,  33, 
and  45  hours.  It  will  be  seen  that  the  larvae  obtained 
when  both  sexual  cells  were  stale  were  of  practically 


108  BLASTOGENIC  VAEIATIONS. 

the  same  size  as  when  they  were  fresh,  even  after  they 
had  been  kept  33  or  45  hours.  The  larvae  from  fresh 
ova  and  stale  sperm,  on  the  other  hand,  were  in  each 
case  distinctly  larger  than  the  normal,  they  differing  on 
an  average  by  '+  5.8  per  cent.,  whilst  those  from  stale 
ova  and  fresh  sperm  were  distinctly  smaller,  they  differ- 
ing by  —  4.9  per  cent.  In  one  of  these  latter  observa- 
tions there  was,  for  some  unknown  reason,  a  slight  in- 
crease in  size,  but  there  can  be  no  doubt  that  on  the 
whole  the  tendency  was  towards  diminution.  On  tak- 
ing means  of  all  the  values  obtained  in  this  and  in  other 
similar  experiments,  it  was  found  that  as  an  average  of 
eight  observations,  the  stale  $  stale  $  larvae  were  di- 
minished by  .7  per  cent. in  size;  as  an  average  of  eleven 
observations,  the  fresh  9  stale  $  larvae  were  increased 
by  4.0  per  cent.,  and  as  an  average  of  ten  observations, 
the  stale  $  fresh  $  larvae  were  diminished  by  6.9  per 
cent.  There  can  be  no  doubt,  therefore,  that  variations 
in  the  degree  of  freshness  of  the  sexual  cells,  that  is  to 
say,  in  the  comparative  state  of  nutrition  of  the  germ- 
plasm  as  a  whole,  do  have  a  very  appreciable  effect  upon 
the  size  of  the  subsequently  developing  larvae.  It  is  to 
be  particularly  noticed  that  the  effect  produced  differs 
entirely  according  to  the  sex-cells  acted  upon,  and  hence 
affords  distinct  evidence  of  the  possibility  that  different 
portions  of  the  same  sex-cell  may  also  react  differently 
to  one  and  the  same  change  of  nutrition. 

Perhaps  a  more  convincing  proof  of  the  influence  of 

the  nutritional  condition  of  the  sex-cells  on  the  offspring 

they  produce  is  afforded  by  certain  other  observations 

on  these  larvae.     On  two  separate  occasions  *  series  of 

*Phil.  Trans.  1895,  B.  p.  585,  and  1898,  B.  p.  483. 


BLASTOGENIC  VARIATIONS. 


109 


artificial  fertilisations  were  carried  out  at  short  inter- 
vals over  periods  of  several  months,  and  the  larvse  al- 
lowed to  develop  under  conditions  which  were  probably 
nearly  constant,  except  as  regards  temperature.  For 
this  varying  factor  a  correction  could  be  easily  applied. 
In  spite  of  the  constancy  of  environmental  conditions, 
however,  the  size  of  the  larvss  showed  very  marked 


Apr.   Apr.     M  y  June  July  July    Aug.  Sept.  Oct.    Nov.   Mov.    Dec.  Jan. 
Ifet    35th     19th    12th     6th  .80th,  23cd    16th   JOth    Scd    jWih     ;30th  13th 

FIG.  19. — Seasonal  variation  in  size  of  larvae. 

variations.  The  range  of  these  variations  may  be 
gathered  from  the  accompanying  diagram.  Here  the 
ordinates  represent  the  mean  body  lengths  of  the  larvse 
in  micrometer  eyepiece  units  (of  which  152.3  are 
equivalent  to  1  mm.),  and  the  abscissas  the  time  of  year 
at  which  the  fertilisations  were  made.  It  will  be  seen 
that  in  April  and  May  the  larvae  were  on  an  average 
about  34  units  in  length,  but  that  then  they  began 
steadily  to  dwindle  down  in  size,  so  that  in  June  they 
were  about  31  units,  and  in  July  and  August  only  about 


110  BLASTOGENIC  VARIATIONS. 

28  units.  From  this  minimum  they  then  rose  rapidly, 
so  that  in  September  they  were  about  32  units,  in  Octo- 
ber and  November  34  units,  and  in  December  and  Janu- 
ary 35  units  in  length.  The  extreme  variations  were 
from  36.80  to  24.49,  the  larvae  of  this  latter  length,  ob- 
tained on  July  9,  being  no  less  than  33.4  per  cent, 
smaller  than  the  former. 

This  extraordinary  seasonal  variation  in  the  size  of 
the  larvae  is  probably  very  closely,  if  not  entirely,  de- 
pendent on  changes  in  the  maturity  and  nutritional  con- 
dition of  the  sexual  products.  Thus,  of  the  specimens  of 
Strongylocentrotus  obtained  in  the  winter  months,  al- 
most every  individual  contained  ripe  sexual  products  in 
large  quantities,  whilst  of  those  obtained  in  the  sum- 
mer months,  not  more  than  about  one  in  four  yielded 
any  ova  or  sperm  at  all  on  shaking  the  ovaries  and 
testes  in  water,  and  occasionally  twenty  or  more  indi- 
viduals were  opened  before  any  ripe  sperm  was  ob- 
tained. Again,  the  best  of  the  specimens  obtained  in 
the  summer  months  did  not  contain  nearly  so  much  of 
the  ripe  sexual  products  as  they  did  in  the  winter. 

That  this  effect  of  season  on  the  condition  of  the  sex- 
cells  is  more  far-reaching  than  is  implied  in  a  mere 
diminution  of  size  in  the  resulting  offspring,  is  proved 
by  some  observations  on  the  crossing  of  this  species  of 
sea-urchin  with  another  species,  viz.,  8  phcer  echinus 
granularis.  Hybrids  between  Splicer  echinus  ova  and 
Strongylocentrotus  sperm  can  probably  be  obtained, 
though  it  may  be  only  after  several  attempts,  at  all 
times  of  the  year.  It  was  found,  however,  that  their 
structure  was  by  no  means  constant.*  The  majority  of 
*Phil.  Trans.  1898,  B.  p.  470. 


BLASTOGENIC  VARIATIONS.  Ill 

the  hybrids  obtained  in  May,  June,  and  July  were  of  the 
almost  pure  Sphcerechinus  type,  of  which  an  example 
is  given  in  Fig.  20;  but  about  a  third  of  them  or  less 
were  of  the  intermediate  or  Strongylocentrotus  type,  of 
which  an  example  is  given  in  Fig.  21.  In  November, 
on  the  other  hand,  only  about  a  sixth  of  the  hybrid 
larvae  were  of  the  Sphcerechinus  or  maternal  type,  and 


FIG.  20. — Sphcerechinus  FIG.  21. — Sphcerechinus  $  Stron- 

larva.  gylocentrotus   $   larva. 

five-sixths  of  the  paternal  type.  Finally,  in  December 
and  January,  all  the  larvae  were  of  the  paternal  type. 
These  so-called  paternal  larvae  in  almost  all  cases 
showed  obvious  traces  of  their  hybrid  origin,  but  they 
were  evidently  much  more  inclined  to  the  Strongylocen- 
trotus than  to  the  Sphcerechinus  type. 

Combining  this  series  of  observations  wi'th  that  just 
recorded,  we  therefore  find  that  in  the  summer  months, 
when  the  Strongylocentrotus  sperm  is  in  a  condition  of 
minimum  maturity,  the  Sphcerechinus  $ —  Strongylo- 
centrotus $  hybrids  are  chiefly  of  the  Sphcerechinus 
type.  As,  however,  the  maturity  of  the  sperm  in- 
creases, it  is  able  to  transform  first  a  portion  and  then 


112  BLASTOGENIC  VAKIATIONS. 

the  whole  of  the  hybrid  larvae  from  the  Sphcerechinus 
to  the  Strongylocentrotus  type.  A  repetition  of  these 
crossing  experiments  in  a  subsequent  year  *  confirmed 
the  conclusion  that  the  summer  hybrids  were  more  in- 
clined to  the  Sphcerechinus  type  than  the  winter  ones, 
though  on  this  occasion  they  were  only  very  rarely 
found  to  approach  to  the  pure  Splicer  echinus  type. 

The  reciprocal  cross  of  Strongylocentrotus  ova  with 
Splicer  echinus  sperm  illustrates  still  another  way  in 
which  the  sex-cells  may  be  affected  by  changes  in  ma- 
turity and  nutrition.  Thus  during  April,  May,  and 
June  a  fair  number  of  the  ova  were  cross-fertilised, 
though  no  plutei  were  obtained:  but  in  July  and  August 
some  47  per  cent,  of  the  ova  were  fertilised,  and  29  per 
cent,  of  them  survived  to  the  eight  days  pluteus  stage. 
In  November  and  December,  on  the  other  hand,  with 
one  exception,  not  only  were  no  plutei  obtained,  but,  as 
a  rule,  not  a  single  ovum  was  cross-fertilised.  In  other 
words,  the  Strongylocentrotus  $  Sphcerechinus  $  hy- 
brid is  only  formed  at  a  time  when  the  Strongylocen- 
trotus ova  have  reached  their  minimum  maturity. 

The  observations  made  upon  these  sea-urchin  larvse 
thus  afford  conclusive  evidence  that  changes  in  the  con- 
ditions of  nutrition  of  the  sex-cells  produced  by  keeping 
them  in  sea  water  may  affect  the  size  of  the  larvae  both 
in  a  positive  and  negative  direction,  whilst  changes  of 
nutrition  dependent  on  season  may  produce  a  much 
more  considerable  effect  on  size,  and  may  in  some  in- 
stances so  alter  the  nature  of  the  germ-plasm  as  to  give 
rise  to  most  marked  variations  of  structure  in  the  re- 
sulting hybrid  offspring,  and  in  other  instances  largely 
*  Arch,  f .  Entwickelungsmechanik,  Bd.  ix.  p.  464,  1900. 


BLASTOGENIC  VARIATIONS.  113 

to  abolish  the  normal  resistance  to  cross-fertilisa- 
tion. If  differences  of  nutrition  in  the  parental  germ- 
plasms  as  a  whole  can  produce  such  profound  effects  on 
the  offspring  to  which  they  give  rise,  then,  supposing  it 
is  possible  that  various  individual  portions  of  the  germ- 
plasm  are  capable  of  being  more  or  less  independently 
affected  by  inequalities  of  nutrition,  there  seems  no  rea- 
son to  doubt  that  they  may  show  a  similar  reaction,  and 
so  give  rise  to  variations  in  the  individual  parts  of  an 
organism  which  they  represent  or  "  determine." 

Experiments  upon  higher  organisms  are,  of  course, 
very  much  more  difficult  to  carry  out  than  those  upon 
sea-urchin  larvae,  but  nevertheless  Professor  Ewart  * 
has  been  able  to  bring  to  a  successful  conclusion  some 
experiments  upon  rabbits.  Thus  he  found  "  that  if  a 
well-matured  rabbit  doe  is  prematurely  (i.  e.,  some  time 
before  ovulation  is  due)  fertilised  by  a  buck  of  a  differ- 
ent strain,  the  young  take  after  the  sire;  when  the  fer- 
tilisation takes  place  at  the  usual  time,  some  of  the 
young  resemble  the  buck,  some  the  doe,  whilst  some 
present  new  characters  or  reproduce,  more  or  less  accu- 
rately, one  or  more  of  the  ancestors.  When,  however, 
the  mating  is  delayed  for  about  thirty  hours  beyond  the 
normal  time,  all  the  young,  as  a  rule,  resemble  the  doe. 
It  may  hence  be  inferred  that  in  mammals,  as  in 
echinoderms,  the  characters  of  the  offspring  are  related 
to  the  condition  of  the  germ-cells  at  the  moment  of 
conjugation,  the  offspring  resulting  from  the  union  of 
equally  ripe  germ-cells  differing  from  the  offspring  de- 
veloped from  the  conjugation  of  ripe  and  unripe  germ- 

*  Presidential  Address  before  the  British  Association.     Vide  Na- 
ture, vol.  Ixiv,  p.  482,  1901. 


114  BLASTOGENIC  VAEIATIONS. 

cells,  and  still  more  from  the  union  of  fresh  and  over- 
ripe germ-cells."  Upon  plants,  as  far  as  I  am  aware, 
no  direct  experiments  have  been  made,  but  some  obser- 
vations of  De  Vries  *  upon  (Enothera  Lamarckiana 
bear  closely  upon  the  matter.  Some  seeds  of  this  plant 
had  been  kept  for  5J  years  before  sowing,  and  it  was 
then  found  that  only  about  1  per  cent,  of  them  germi- 
nated, instead  of  the  usual  14  per  cent,  or  so.  Of  the 
seedlings  obtained,  however,  about  40  per  cent,  were 
"  mutations,"  whilst  the  proportion  of  mutations  ob- 
tained from  fresh  seeds  was  only  1  to  5  per  cent. 

Weismann's  second  factor  in  the  production  of  varia- 
tions is  the  so-called  amphimixis,  or  sexual  reproduction 
in  multicellular  organisms,  and  conjugation  in  unicel- 
lular. That  this  is  one  of  the  chief  causes  of  variation 
was  maintained  by  W.  K.  Brooks  f  some  years  ago. 
Basing  his  theory  on  Darwin's  hypothesis  of  Pan- 
genesis,  he  considered  that  as  every  "  gemmule  "  of  the 
spermatozoon  united  with  that  particle  of  the  ovum 
which  is  destined  to  give  rise  in  the  offspring  to  the  cell 
which  corresponds  to  the  one  which  produced  the  germ 
or  gemmule,  then  such  a  cell  will  be  a  hybrid,  and  will 
therefore  tend  to  vary.  In  his  opinion  the  egg-cell  is 
the  conservative  principle  which  controls  the  transmis- 
sion of  purely  racial  or  specific  characters,  whereas  the 
sperm  cell  is  the  progressive  element  which  causes 
variation. 

To  what  extent  are  we  justified  in  assuming  that  this 
process  of  amphimixis  does  furnish  an  inexhaustible 

*  ••  Die  Mutationstheorie,"  p.  360. 

f  "  The  Law  of  Heredity:  A  Study  of  the  Cause  of  Variation  and 
the  Origin  of  Living  Organisms,"  Baltimore,  1883. 


BLASTOOENIC  VARIATIONS.  115 

supply  of  fresh  combinations  of  individual  variations,  as 
Weismann  maintains?  Exact  numerical  evidence  upon 
the  point  is,  indeed,  almost  entirely  wanting,  but  one's 
own  everyday  experience  is  all  in  favour  of  its  validity. 
Thus  one  knows  that  animals  of  the  same  litter,  which 
during  embryonic  development  must  have  been  exposed 
to  very  nearly  equal  environmental  conditions,  differ  al- 
most as  much  from  each  other  as  from  animals  of  for- 
mer litters,  and  in  many  cases  not  very  much  less 
than  from  animals  in  the  litters  of  entirely  different 
parents. 

ISTow,  as  this  phenomenon  is  one  of  almost  universal 
occurrence,  it  cannot  be  maintained  that  the  observed 
variations  may  be  brought  about  by  chance  differences 
of  environmental  conditions  acting  during  development. 
They  must  obviously  be  in  chief  part  the  result  of  dif- 
ferences in  the  individual  sex-cells  from  which  the  off- 
spring took  their  rise.  So  important  is  this  conclusion 
that  it  was  enunciated  by  Victor  Hensen  as  a  funda- 
mental law  of  amphigonic  heredity.  This  has  been 
thus  worded  by  Weismann:*  "  The  individual  is  deter- 
mined at  the  time  of  fertilisation,  or,  in  other  words, 
the  individuality  of  an  organism  results  from  the  fact 
that  the  germ-plasm  is  composed  of  the  paternal  and 
maternal  ids  which  are  brought  together  in  the  egg- 
cell." 

Very  interesting  evidence  in  favour  of  this  law  is 
furnished  by  cases  of  identical  human  twins.  It  has 
long  been  known  that  whilst  the  larger  number  of 
twins  show  no  greater  resemblance  to  each  other  than 
do  children  of  the  same  parents  born  consecutively,  a 
*  "  Germ-plasm,"  p.  253. 


116  BLASTOGENIC  VARIATIONS. 

certain  proportion  exhibit  a  most  striking  resemblance, 
which,  although  not  perfect,  is  much  closer  than  has 
ever  been  observed  in  children  born  successively. 
These  Weismann  speaks  of  as  "  identical  "  twins.  He 
says  there  is  every  reason  to  suppose  that  such  twins  are 
derived  from  a  single  ovum  and  spermatozoon,  whilst 
dissimilar  twins  are  derived  from  two  ova,  which  must, 
of  course,  have  been  fertilised  by  two  different  sperma- 
tozoa. If  this  is  actually  the  case,  it  furnishes  a  proof 
that  heredity  is  potentially  decided  at  the  time  of  ferti- 
lisation. 

Interesting  cases  of  identical  twins  have  been  re- 
corded by  Galton  in  his  book  on  "  Inquiries  into  Hu- 
man Faculty."  With  reference  to  disease,  for  instance, 
it  was  found  that  both  twins  were  apt  to  sicken  at  the 
same  time  in  9  out  of  the  35  cases  collected.  Either 
their  illnesses  were  non-contagious,  or,  if  contagious,  the 
twins  caught  them  simultaneously.  The  mental  and 
moral  resemblance  between  the  twins  was  just  as  close 
as  the  physical.  An  instance  cited  by  Dr.  J.  Moreau  * 
well  illustrates  this.  His  case  was  one  of  twin  brothers 
who  had  been  confined  on  account  of  monomania.  They 
were  physically  so  alike  as  to  be  easily  mistaken  for 
one  another,  and  as  regards  their  moral  condition  they 
had  exactly  the  same  dominant  ideas;  they  both  con- 
sidered themselves  subject  to  the  same  imaginary  per- 
secutions ;  they  both  had  hallucinations  of  hearing ;  both 
were  melancholy  and  morose. 

Unfortunately,  Galton  did  not  obtain  any  exact 
anthropometric  data.  Weismann  has  obtained  one 
series  of  measurements,  however,  viz.,  for  twin  brothers 
*  "Psychologic  Morbide,"  Paris,  1859,  p.  172. 


BLASTOGENIC  VARIATIONS.  117 

seventeen    years    of    age.*      The    following    are    the 
measurements  made: 

PER  CENT. 
TWIN  A.        TWIN  B.      DIFFERENCE. 

Stature, 172cm.  170cm.  1.2 

Left  arm, 74  74  0.0 

Right  arm 71  74  4.1 

Left  upper  arm,        ....  27  27.5  1.8 

Forearm 27  26  3.8 

These  slight  differences  are  probably  due  to  the  effect 
of  external  influences  acting  during  the  course  of  de- 
velopment, or  are  somatic,  as  distinguished  from  blas- 
togenic,  variations. 

In  a  series  of  measurements  on  twin  brothers 
obtained  by  the  author,  the  resemblance  was  very  much 
closer,  the  difference  in  no  case  reaching  even  1  per  cent. 
These  brothers,  aged  twenty-three,  were  extraordi- 
narily alike  in  physiognomy,  and,  moreover,  they 
had  both  suffered  at  the  same  times  from  the  same 
diseases,  viz.,  bronchitis,  measles,  chicken-pox,  mumps, 
and  influenza.  The  slightly  smaller  one  of  the  two 
had  had  a  rather  more  severe  attack  of  bronchitis  than 
his  brother,  when  a  year  and  a  half  old,  and  so,  perhaps, 
but  for  this,  the  physical  resemblance  would  have  been 
even  closer: 

PER  CENT. 
TWIN  A.  TWIN  B.       DIFFERENCE. 

Standing  height 173.00cm.  172.67cm.     -.19 

Sitting  height  (from  seat  of  chair),  88.03  87.87  —.18 

Span  of  arms,         ....  179.90  179.88  -.01 

Length  of  right  mid-finger  (from 

metacarpo-phalangeal  joint),  10.99  10.98  —.09 

Span  of  hand,         ....  21.33  21.30  -.14 

Length  of  skull  (occipital  protuber- 
ance to  base  of  nose),  .        .  18.52  18.42  —.54 

Maximum  breadth  of  skull,   .        .  15.13  15.01  —.80 
f  "Germ-plasm,"  p.  253. 


118  BLASTOGENIC  VARIATIONS. 

The  finger  prints,  though  bearing  some  resemblance, 
were  nevertheless  easily  distinguishable.  Hence  in 
this  case  Galton's  finger-print  method  would  serve  for 
an  identification,  whilst  Bertillon's  anthropometric  sys- 
tem would  be  useless. 

The  very  slight  modifications  produced  in  these  twins 
by  the  action  of  environment  during  growth  is  prob- 
ably explained  by  the  fact  that  they  had  always  been 
brought  up  together,  and  so  exposed  to  practically  the 
same  conditions  all  their  lives. 

In  another  series  of  measurements  upon  twin 
brothers  (aged  twelve),  the  differences  observed  were 
somewhat  greater,  and  the  facial  resemblance  was  like- 
wise not  quite  so  marked  as  in  the  previous  case.  The 
boys  had  both  had  scarlet  fever,  chicken-pox,  and 
measles  at  the  same  times.  Of  the  measurements  given, 
it  will  be  seen  that  the  span  of  arms  and  length  of  fore- 
arm showed  the  greatest  differences.  As  before,  one 
twin  was  slightly  smaller  than  the  other  in  respect  of 
every  measurement  made : 

PER  CENT. 
TWIN  A.       TWIN  B.   DIFFERENCE. 

Standing  height 143.22cm.  142.62cm.  -  .42 

Sitting  height 72.93  72.87  -  .08 

Span  of  arms 150.73  148.81  -1.27 

Elbow  to  tip  of  mid-finger,   .     .     .      39.59  38.93  —1.67 

Length  of  right  mid-finger,  .     .     .      10.06  10.03  —  .30 
Circumference  of   head  (over  occi- 
pital protuberance  and  2.5  cm. 

above  eyebrows),     ....      50.23  49.97  —  .52 

From  the  vegetable  kingdom  Weismann  also  quotes 
an  instance  in  support  of  Hensen's  law.  Thus,  in  re- 
spect of  two  species  of  Oxalis,  "  the  flowers  of  the  dif- 
ferent hybrids  were  by  no  means  quite  similar,  but 


BLASTOGENIC  VARIATIONS.  119 

three  principal  forms  could  be  distinguished  according 
to  the  combination  of  colours  in  the  flowers.  The 
flowers  of  the  same  hybrid,  however,  resembled  each 
other  in  the  most  minute  details.  One  plant,  for  in- 
stance, had  violet  petals  of  a  rather  pinker  tint  than 
those  of  one  of  the  parent  species,  and  all  the  petals 
were  strongly  tinged  with  red  on  one  and  the  same 
lateral  margin.  As  far  as  I  could  observe,  all  the 
flowers  were  similarly  coloured  on  this  stock.  On  an- 
other stock,  all  the  sepals  had  brown  rims,  and  on  a 
third  there  was  a  narrow  dark  orange-coloured  band  in 
the  centre  of  each  flower.  In  these  cases,  therefore, 
the  combination  of  the  colours  of  the  parents  which  ap- 
peared in  the  petals  of  the  hybrids  must  have  been  de- 
cided at  the  time  of  fertilisation."  * 

That  the  influence  which  the  maternal  fluids  canj 
exert  on  an  embryo  during  intra-uterine  development  is  f 
at  best  very  slight  seems  at  first  sight  to  be  proved  by 
the  experiments  of  Heape  f  on  the  transplantation  of 
rabbits'  ova.     In  the  first  successful  experiment,  two 
segmenting  ova  were  obtained  from  an  Angora  doe  rab-^ 
bit  which  had  been  fertilised  by  an  Angora  buck  thirty- 
two  hours  previously,  and  were  immediately  transferred 
to  the  upper  end  of  the  fallopian  tube  of  a  Belgian  hare 
rabbit  which  had  been  fertilised  three  hours  before  by 
a  buck  of  the  same  breed    as  herself.     In  due  course 
this  Belgian  hare  doe  gave  birth  to  six  young.     Four 
of  these  resembled  herself  and  her  mate,  but  the  other 
two  were  undoubted  Angoras.      The  Angora  young 
were  characterised  by  the  possession  of  the  long  silky 

*  "  Germ-plasm,"  p.  256. 

fProc.  Roy.  Soc.,  xlviii.  p.  457,  1890. 


120  BLASTOGENIC  VARIATIONS. 

hair  peculiar  to  the  breed,  and  were  true  albinoes,  like 
their  Angora  parents.  They  also  possessed  the  char- 
acteristic habit  of  slowly  swaying  the  head  from  side  to 
side  when  they  looked  at  one.  Both  of  the  Angora 
young  were  born  bigger  and  stronger  than  any  of  the 
other  young,  and  they  all  along  maintained  their  su- 
premacy in  this  direction.  Heape  could  observe  no 
sign  in  the  Angora  young  of  any  Belgian  hare  strain, 
and  the  Belgian  hare  young  showed  no  likeness  to  their 
foster-brothers. 

In  a  subsequent  paper,*  Heape  records  another  suc- 
cessful experiment.  In  this  a  Belgian  hare  doe  was 
covered  by  a  Belgian  hare  buck,  and  shortly  after  the 
segmenting  ova  obtained  from  a  Dutch  doe  which  had 
been  covered  by  a  Dutch  buck  twenty-four  hours  pre- 
viously were  transferred  to  her  fallopian  tube.  The 
Belgian  hare  doe  gave  birth  to  seven  young,  of  which 
five  Avere  Belgian  hares,  and  two  very  irregularly 
marked  Dutch.  It  was  found,  however,  on  putting  the 
same  Dutch  buck  which  had  been  used  in  this  experi- 
ment to  a  thoroughbred  Dutch  doe,  most,  if  not  all,  of 
the  litter  resulting  were  as  badly  marked  as  the  Dutch 
foster-children.  Hence  it  is  not  necessary  to  suppose 
that  the  foster-mother  was  the  cause  of  the  irregularity. 
From  this  and  other  evidence  Heape  considers  he  is 
justified  in  concluding  that  the  uterine  foster-mother 
exerts  no  modifying  influence  upon  her  foster-children, 
in  so  far  as  can  be  tested  by  the  examination  of  a  single 
generation.  Romanes  t  has  however  remarked,  that 
inasmuch  as  rabbits,  when  crossed  in  the  ordinary  way, 

*Proc.  Roy.  Soc.,  Ixii.  p.  178, 1897. 

f  "Darwin  and  after  Darwin,"  vol.  ii.  p.  146. 


BLASTOGENIC  VARIATIONS.  121 

never  throw  intermediate  characters,  the  result  of 
Heape's  experiment  is  without  significance,  as  far  as  it 
bears  on  the  inheritance  of  acquired  characters.  Heape 
considers  that  Romanes  is  mistaken  in  this  view,  for  he 
has  himself  obtained  experimental  evidence  to  show 
that  some  of  the  young  obtained  by  crossing  are  of  an 
intermediate  character.  It  is  nevertheless  true  that  in 
the  majority  of  cases  the  young  are  apparently  pure 
bred  of  one  type  or  the  other,  and  hence  the  value  which 
ought  to  attach  to  Heape's  experiments,  so  far  as  they 
relate  to  the  production  of  somatic  variations  by  change 
of  environmental  conditions  during  embryonic  develop- 
ment, is  probably  not  very  great. 

The  evidence  so  far  available  seems  to  render  it 
highly  probable,  therefore,  that  the  major  part  of  the 
variation  exhibited  by  organisms  is  of  blastogenic 
rather  than  somatic  origin.  It  is  due  more  to  dif- 
ferences in  the  germ  cells  than  to  external  influences 
acting  during  ontogeny.  If  it  be  found  possible  to  col- 
lect considerable  series  of  anthropometric  measure- 
ments of  identical  twins,  and  to  compare  them,  as  re- 
gards variability,  with  similar  measurements  on  dissimi- 
lar twins,  then  we  may  hope  to  obtain  some  adequate 
conception  as  to  what  proportion  of  the  variation  ex- 
hibited by  adult  individuals  is  due  to  external  influences 
acting  during  pre-  and  post-natal  existence.  Also  by 
comparing  the  variability  of  dissimilar  twins  with  that 
of  members  of  families  produced  in  the  normal  manner 
of  one  at  a  birth,  we  may  hope  to  determine  what 
changes,  if  any,  are  produced  by  the  slight  differences 
in  the  maternal  fluids  which  must  doubtless  exist  dur- 
ing the  development  of  the  different  offspring. 


122  BLASTOGENIC  VARIATIONS. 

The  variations  of  offspring  are  therefore  largely  pro- 
duced by_  the  mingling  of  %  dissimilar  parental  germ- 
plasms,  so  that  the  offspring  do  not  closely  resemble 
either  each_  othert  or  their  parents.  But  there  must 
clearly  be  a  relation  of  some  sort  between  them.  As 
to  the  extent  of  this  relation,  we  are  chiefly  indebted 
for  our  knowledge  to  the  labours  of  Mr.  Francis  Galton. 
In  his  work  on  "  Natural  Inheritance,"  he  has  analysed 
a  very  large  number  of  anthropometric  data  which  were 
collected  by  himself  specially  for  the  purpose.  The 
most  important  of  them  consist  of  records  of  the 
stature,  eye-colour,  artistic  faculty,  and  condition  of 
health  of  the  various  members  of  some  150  distinct 
families,  extending  over  three  or  more  generations. 
Arguing  from  these  data,  he  concluded  that  on  an  aver- 
age each  parent  contributed  to  the  characters  of  his  or 
her  offspring  J  of  their  amount,  or  both  parents  to- 
gether contributed  a  half;  whilst  each  grandparent  con- 
tributed T\,  or  the  four  grandparents  together  J,  and  so 
on;  but  he  considered  his  data  insufficient  to  warrant 
him  in  extending  the  sequence  to  more  distant  genera- 
tions. 

Some  years  later,  Galton  obtained  other  more  fav- 
ourable data.*  These  enabled  him  to  ascertain  the  con- 
tributions of  ancestors  to  offspring  with  much  greater 
exactness,  and  warranted  him  in  formulating  a  Law  of 
Ancestral  Heredity,  which  there  is  some  reason  for 
thinking  may  prove  to  be  universally  applicable  to 
bisexual  descent.  The  data  consisted  in  long  series  'of 
records  of  the  colours  of  a  large  pedigree  stock  of 
Basset  hounds,  extending  through  many  successive 
*Proc.  Roy.  Soc.,  vol.  Ixi.  p.  401,  1897. 


BLASTOGENIC  VARIATIONS.  123 

generations.  These  records  were  preserved  by  Sir 
Everett  Millais,  who  had  originated  the  stock  of  hounds 
some  twenty  years  ago.  The  Bassets  are  dwarf  blood- 
hounds, of  two  and  only  two  recognised  varieties  of 
colour.  They  are  either  white  with  large  blotches 
ranging  between  red  and  yellow,  or  they  may,  in  addi- 
tion, be  marked  with  more  or  less  black.  In  the  former 
case  they  are  technically  known  as  "  lemon  and  white," 
and  in  the  latter  case  as  "  tricolour."  Transitional 
cases  between  these  two  forms  are  very  rare.  No  less 
than  817  hounds  of  known  colour,  all  descended  from 
parents  of  known  colour,  were  available  as  material. 
In  567  out  of  these  817  the  colours  of  all  four  grand- 
parents were  known,  and  in  188  cases  the  colours  of  all 
eight  great-grandparents  were  known  as  well.  It 
was  found  that  79  per  cent,  of  the  parents  of  tricolour 
hounds  were  tricolour,  whilst  56  per  cent,  of  the 
parents  of  lemon  and  white  hounds  were  tricolour. 
Hence  from  these  values  the  contributions  of  unknown 
ancestors  could  easily  be  calculated.  Working  from 
these  numerous  data,  Galton  was  able  to  confirm  en- 
tirely his  previous  conclusions  regarding  heredity,  and 
extend  them  in  the  direction  then  hinted  at.  He  proved 
that  the  two  parents  do  contribute  between  them  one- 
half  or  (0.5)  of  the  total  heritage  of  the  offspring: 
whilst  the  four  grandparents  contribute  one-quarter,  or 
(0.5)2:  the  eight  great-grandparents  one-eighth,  or 
(0.5)3,  and  so  on.  Thus  the  sum  of  the  ancestral  con- 
tributions is  expressed  by  the  series  [  (0.5)  +  (0.5)2  ;-f- 
(0.5)3  +  .  .  .  etc.],  which,  being  equal  to  1,  accounts 
for  the  whole  heritage.  The  same  statement  may  be 
put  in  a  different  form,  by  saying  that  each  parent  con- 


124  BLASTOGENIC  VAEIATIONS. 

tributes  on  an  average  one-quarter,  or  (0.5)2,  each 
grandparent  one-sixteenth,  or  (0.5)4,  and  so  on,  or  that 
the  occupier  of  each  ancestral  place  in  the  nth  degree, 
whatever  be  the  value  of  n,  contributes  (0.5)2n  of  the 
heritage. 

It  is  unnecessary  to  quote  the  numerical  details  ad- 
duced by  Galton,  but  two  final  results  may  be  men- 
tioned just  to  show  how  close  was  the  approximation  be- 
tween fact  and  theory.  Thus  in  one  series  387  tri- 
colour offspring  were  obtained  from  certain  parents  of 
known  colour,  themselves  the  offspring  of  parents  of 
known  colour.  On  the  law  of  heredity,  the  number  of 
tricolour  offspring  should  have  been  391.  In  the 
other  series,  the  colours  of  the  great-grandparents  were 
known  in  addition,  and  in  this  case  the  approximation 
was  even  closer.  One  hundred  and  eighty-one  tri- 
colour offspring  were  obtained,  as  against  the  calculated 
number  of  180. 

Galton  points  out  that  there  is  nothing  in  his  statis- 
tical law  to  contradict  the  generally  accepted  view  that 
the  chief,  if  not  the  sole,  line  of  descent  runs  from  germ 
to  germ,  and  not  from  person  to  person.  The  person 
on  the  whole  may  be  accepted  as  a  fair  representative 
of  the  germ,  and  so  statistical  laws  which  apply  to  per- 
sons would  apply  to  germs  also.  Now  the  law  is 
strictly  consonant  with  the  '  observed  binary  subdivi- 
sions of  the  germ  cells,  and  the  concomitant  extrusion 
and  loss  of  one-half  of  the  several  contributions  from 
each  of  the  two  parents  to  the  germ  cell  of  the  off- 
spring. 

Galton's  law  has  been  shown  by  Pearson  *  to  be  even 
*Proc.  Roy.  Soc.,  Ixii.  p.  386, 


BLASTOGENIC  VARIATIONS.  125 

more  fundamental  and  far-reaching  than  its  author 
claimed  it  to  be.  Thus  he  says,  "  If  Mr.  Galton's  law 
can  be  firmly  established,  it  is  a  complete  solution,  at 
any  rate  to  a  first  approximation,  of  the  whole  problem 
of  heredity."  Professor  Pearson  points  out  that  by 
means  of  it  we  are  enabled  to  find  the  coefficients  of  cor- 
relation between  an  individual  and  any  individual  an- 
cestor, and  that  these  coefficients  in  their  turn  will 
suffice  to  determine  all  inheritance,  whether  direct  or 
collateral. 

As  regards  the  relation  of  this  law  of  heredity  to 
variations  produced  by  amphimixis,  it  is  necessary  to 
emphasize  one  fact,  viz.,  that  it  concerns  only  the  j 
average  contributions  of  ancestors  to  offspring,  and  not  \ 
the  absolute  contributions.  Within  what  limits  the 
contributions  of  each  parent  and  grandparent  to  the 
heritage  of  a  child  may  vary,  nothing  whatsoever  is 
known.  It  is  possible  that  they  may  be  very  wide  in- 
deed, and  everyday  experience  tends  to  give  colour  to 
this  view.  How  trite  is  the  expression  that  such  and 
such  a  child  is  the  "  image  "  of  his  father  or  mother, 
whilst  instances  are  no  less  common  in  which  it  is  diffi- 
cult, if  not  impossible,  to  trace  any  distinct  resem- 
blance between  parent  and  child.  Such  cases  as  these, 
even  if  they  could  be  substantiated  by  exact  physical 
measurements,  would  in  reality  prove  but  little.  It 
would  be  impossible  to  make  accurate  comparisons  of 
all  the  tissues  and  organs  of  the  body,  and  of  the  cells 
composing  them,  and  it  might  be  that  these  unexamined 
and  unexaminable  portions  of  the  organism  in  reality 
possessed  a  very  close  correlation  with  the  correspond- 
ing parental  tissues.  The  average  degree  of  correla- 


126  BLASTOGENIC  VARIATIONS. 

tion  for  all  the  tissues  in  the  body  might  thus  be  just  as 
great  as  for  an  individual  who  to  all  appearances  closely 
resembled  his  parents,  but  in  this  case  chiefly  in  exter- 
nal characters  and  not  internal. 

If  amphimixis  be  so  largely  responsible  for  the  varia- 
tions observed  in  offspring,  what,  then,  are  the  rules 
which  govern  the  amount  and  range  of  these  variations  ? 
For  the  answer  to  this  question  we  are  again  primarily 
indebted  to  the  labours  of  Mr.  Galton.  He  set  himself 
to  determine  the  exact  average  relationships  between 
the  two  parents  and  their  offspring.  It  might  be 
thought  that  this  was  so  simple  and  obvious  as  to  ren- 
der it  waste  of  time  to  put  it  to  the  test  of  experiment. 
It  might  be  thought,  in  fact,  that  the  average  char- 
acters of  offspring  are  a  mean  between  those  of  the 
parents.  But  this  is  far  from  being  the  case.  As  Gal- 
ton  first  showed,  by  means  of  extensive  observations  on 
the  size  of  sweet-pea  seeds  obtained  from  plants  which 
had  been  grown  from  seeds  of  known  size,  the  average 
characters  of  the  offspring  show  a  considerable  regres- 
sion towards  the  mean  characters  of  the  race.  That  is 
to  say,  in  the  present  instance,  the  size  of  the  filial  seeds 
was,  on  an  average,  more  mediocre  than  that  of  the 
parent  seeds.  By  means  of  his  data,  above  referred  to, 
concerning  the  stature  of  families,  Galton  was  able  not 
only  to  thoroughly  substantiate  this  phenomenon  of  re- 
gression, but  to  calculate  with  some  degree  of  exactness 
the  actual  amount  of  regression  occurring  between 
various  kinsmen.  For  example,  he  found  that  if 
parents  were  sorted  into  groups  according  to  their 
stature,  then  the  stature  of  their  sons,  on  an  average, 
deviated  only  two-thirds  as  much  from  the  mean  stature 


1    UN 


BLASTOGENIC  VARIATIONS.  127 

of  the  general  population  as  theirs  did.  Thus,  if  the 
mid-parental  stature  (the  average  between  the  statures 
of  the  man  and  the  transmuted  stature  of  the  woman) 
be  72  inches,  or  3|  inches  greater  than  the  mean  stature 
of  the  whole  population,  then  the  average  stature  of 
their  sons  will  be  only  3f  X  f  =  2^  inches  greater  than 
the  mean.  If  the  mid-parental  stature  be  66  inches,  or 
2J  inches  less  than  the  mean  stature,  then  the  average 
filial  stature  will  be  66|  inches,  or  only  1£  inches  less 
than  the  general  mean  stature.  In  addition  to  calculat- 
ing the  regression  between  parents  and  sons,  and  grand- 
parents and  grandsons,  Galton  calculated  it  for  col- 
lateral relationships,  as  between  uncles  and  nephews, 
and  brothers  and  brothers. 

Many  of  the  data  recorded  by  Galton  in  his  "  Natural 
Inheritance  "  were  worked  over  again  by  Pearson  in 
his  memoir  on  "  ^Regression,  Heredity,  and  Pan- 
mixia/' *  and  various  improvements  in  statistical 
methods  suggested.  The  mathematical  measure  of  re- 
gression, or  coefficient  of  regression,  he  defined  to  be 
"  the  ratio  of  mean  deviation  of  offspring  of  selected 
parents  from  the  mean  of  all  the  offspring  to  the  devia- 
tion of  the  selected  parents  from  the  mean  of  all  the 
parents."  It  is  to  be  noticed  that  according  to  this 
definition,  the  deviation  of  the  offspring  ought  to  be 
measured  from  the  mean  of  the  offspring  of  the  general 
population,  and  not  of  the  whole  population,  both 
parents  and  offspring,  for  thereby  factors  such  as  secu- 
lar natural  selection  and  reproductive  selection  are 
allowed  for. 

In  this  memoir,  when  discussing  coefficients  of  re- 
*Phil.  Trans.  1896,  A.  p.  253. 


128  BLASTOGENIC  VAKIATIONS. 

gression,  Pearson  came  to  the  conclusion  that  there  was 
not  at  that  time  sufficient  ground  for  forming  any  defi- 
nite conclusion  as  to  the  manner  in  which  lineal  and  col- 
lateral heredity  were  related.  Thus  it  did  not  appear 
necessary  to  him  that  the  coefficient  of  the  former 
should  be  half  that  of  the  latter,  as  Galton  had  sup- 
posed. On  attacking  the  problem  a  second  time,* 
however,  Pearson  succeeded  in  proving  that  they  were 
connected,  according  to  a  mathematically  ascertainable 
relationship,  so  that,  starting  from  Galton's  law  of 
heredity,  it  was  possible  to  calculate  the  coefficients  of 
regression  or  correlation  between  an  individual  and  any 
of  his  kinsmen,  either  direct  or  collateral.  Thus  Pear- 
son calculated  the  coefficient  of  regression  between  mid- 
parent  and  son  to  be  .6,  or  somewhat  less  than  that 
found  by  Galton.  Between  a  single  parent  and  a  son, 
it  would,  therefore,  be  .3.  Between  grandparent  and 
grandson  it  was  .15,  between  great-grandparent  and 
great-grandson  .075,  and  so  on.  Between  brothers  it 
was  .4,  or  considerably  less  than  the  coefficient  found  by 
Galton.  Nevertheless  this  value  confirms  Galton's 
conclusion  that  brothers  are  more  closely  related  to 
each  other  by  blood  than  are  fathers  and  sons. 

It  may  be  pointed  out  that  in  a  stable  population  the 
coefficients  of  regression  and  of  correlation  between  an 
individual  and  an  ancestor  are  one  and  the  same  thing. 
If,  however,  the  population  is  not  stable,  so  that  the 
variability  of  the  offspring  differs  from  the  variability 
of  the  parents,  then  these  coefficients  also  differ  slightly. 

The  importance  of  this  extension  of  Galton's  law  can- 
not be  rated  too  highly,  for  by  its  means  the  whole 
*Proc.  Roy.  Soc.,  Ixii.  p.  386. 


BLASTOGENIC  VAEIATIONS.  129 

theory  of  heredity  is  rendered  simple,  straightforward, 
and  luminous.  Pearson  points  out  that  we  no  longer 
need  to  know  the  characters  of  parents,  grandparents, 
etc.,  to  test  the  law,  for  any  single  relationship,  near  or 
far,  direct  or  collateral,  will  bring  its  cjuota  of  evidence 
for  or  against  it. 

Galton's  principle  of  "  Eegression  towards  Medi-J 
ocrity  "  has  been  spoken  of  occasionally  as  if  it  werd| 
something  abnormal  and  unexpected;  something,  in- 
deed, unexplained  and  inexplicable.  It  is  clearly  noth- 
ing of  the  kind,  however,  but  only  what  might  readily 
be  deduced  from  his  law  of  Ancestral  Heredity,  sup- 
posing that  this  and  this  alone  were  known  to  us. 
Thus  we  have  seen  that  offspring  derive  certain  por- 
tions of  their  heritage  from  their  grandparents  and 
more  remote  ancestors,  and  as  these  are  likely  to  be,  on 
an  average,  more  mediocre  than  their  parents,  they 
water  down  the  parental  characters  transmitted  to  the 
offspring.  Supposing  all  the  grandparents  and  more 
remote  ancestors  of  any  given  parents  were  absolutely 
mediocre,  then,  as  the  offspring  receive  only  half  their 
heritage  from  these  parents,  they  would  exhibit  their 
characters  in  only  half  strength,  or  the  coefficient  of 
regression  would  be  .5,  and  not  .6.  The  reason  why 
the  regression  reaches,  on  an  average,  the  higher  figure, 
is  of  course  that  the  grandparents  and  other  ancestors 
are  not,  as  a  rule,  absolutely  mediocre.  They  possess 
the  characters  exhibited  by  the  parents,  though  in  a 
diminished  degree.  Grandparents  regress  on  parents 
to  just  the  same  extent  as  offspring  do. 

It  may  perhaps  be  enquired  how  it  is  that,  if  off- 
spring are  on  an  average  more  mediocre  than  their 


130 


BLASTOGENIC  VAEIATIONS. 


parents,  the  variability  of  the  race  does  not  become  less 
and  less  for  each  generation,  and  so  finally  be  reduced 
to  zero.  Why  this  is  not  the  case  is  perhaps  most 
readily  grasped  by  examining  a  statistical  table  of  the 
relations  between  parent  and  offspring  in  respect  of 
some  character.  The  table  here  given  is  reduced  from 
a  larger  one  given  by  Galton  in  his  "  Natural  Inherit- 
ance "  (p.  208),  and  represents  the  numbers  of  adult 
children  of  various  heights  born  of  205  mid-parents  of 
various  heights.  For  instance,  we  see  that  the  17  mid- 


M 

fiS 

egi 

ti  5  w 

NUMBER  OF  ADULT  CHILDREN  OP  HEIGHTS 

*i 

«? 

n 
*% 

ill 

Si* 

below 
63 

63-65 

65-67 

67-69 

69-71 

71-73 

73  & 
over 

MEDIAl 
HEIGH1 
CHILDB 

5 

73  &  over 

1 

3 

17 

71-73 

4 

8 

18 

22 

10 

70.6 

63 

69-71 

1 

18 

23 

90 

42 

15- 

69.1 

82 

67-69 

4 

37 

92 

131 

126 

37 

3 

67.9 

32 

65-67 

4 

22 

37 

49 

29 

3 

67.0 

6 

below  65 

3 

14 

9 

8 

3 

65.6 

parents  71  to  73  inches  in  height  had  between  them  62 
children,  of  whom  the  most  frequently  occurring  were 
also  71  to  73  inches,  or  the  same  height  as  their  parents. 
Children  of  from  69  to  71  inches  were,  however,  nearly 
as  frequent,  so  that,  on  the  whole,  it  is  obvious  that  the 
stature  of  the  children  was  more  mediocre  than  that  of 
the  parents.  The  median,  or  middle  height  of  all  these 
62  values,  was,  in  fact,  only  70.6  inches,  or  1.4  inch 
less  than  the  median  height  of  the  mid-parents.  The 
median  of  the  children  of  mid-parents  69  to  71  inches 
in  height,  was  .9  inch  less  than  their  median;  whilst  in 
the  children  of  mid-parents  varying  from  67  to  69 
inches  it  was  only  .1  inch  less,  for  the  median  of  these 


BLASTOGENIC  VARIATIONS.  131 

mid-parents,  viz.,  68  inches,  was  very  nearly  that  of  the 
whole  population,  and  so  obviously  the  filial  height 
could  undergo  no  regression,  but  would  be  practically 
the  same  value.  The  values  in  this  table  thus  illus- 
trate the  existence  of  regression,  but  they  also  indicate 
that  the  offspring  produced  by  these  mid-parents  are,  as 
a  whole,  no  less  variable  than  they  themselves  are.  The 
offspring  are,  in  fact,  more  variable,  as  a  mid-parental 
stature,  being  the  mean  of  two  parental  statures,  is  ob- 
viously, on  an  average,  less  variable  than  either  stature 
individually.  Thus  the  mid-parents  vary  roughly  be- 
tween about  74  and  64  inches,  but  the  children  between 
75  and  62  inches.  This  table  therefore  teaches  us  that 
though  the  children  are,  on  an  average,  more  mediocre 
than  their  parents,  yet  the  general  variability  of  the 
race  is  not  diminished.  The  reason  why  the  variability 
remains  undiminished  may  be  seen  by  studying  the  com- 
ponents of  the  vertical  columns  of  the  table.  For  ex- 
ample, with  reference  to  children  71  to  73  inches  in 
height,  we  see  that  mid-parents  of  71  inches  and  up- 
wards contribute  proportionately  more  of  these  tall 
children  than  do  any  other  mid-parents,  but  still  mid- 
parents  of  69  to  71  inches  contribute  (proportionately) 
a  good  many,  and  parents  of  67  to  69  inches  no  small 
number.  Even  mid-parents  of  65  to  67  inches  con- 
tribute a  very  minute  number  of  these  children,  who 
are  thus  no  less  than  6  inches  taller  than  their  parents. 
By  these  several  contributions,  therefore,  the  number 
of  tall — and  similarly  of  other — children  is  kept  up  to 
the  same  level  in  each  generation.  One  may  accord- 
ingly sum  up  the  contents  of  this  table  as  follows:  Tall 
parents  have  many  tall  children,  a  moderate  number 


132  BLASTOGENIC  VARIATIONS. 

of  medium  children,  and  a  very  small  number  of  short 
children;  medium  parents  have  many  medium  children, 
and  moderate  numbers  of  tall  and  short  children;  short 
parents  have  many  short  children,  a  moderate  number 
of  medium  children,  and  a  very  small  number  of  tall 
children. 

As  was  first  pointed  out  by  Mr.  Galton,*  characters 
such  as  stature  and  eye-colour  offer  a  distinct  contrast 
in  their  hereditary  behaviour,  for  whilst  "  Parents  of 
different  statures  usually  transmit  a  blended  heritage 
to  their  children,  parents  of  different  Eye-colours 
usually  transmit  an  alternative  heritage.  .  .  If  one 
parent  has  a  light  Eye-colour  and  the  other  a  dark  Eye- 
colour,  some  of  the  children  will,  as  a  rule,  be  light  and 
the  rest  dark:  they  will  seldom  be  medium  eye-col- 
oured." Thus  eye-colour  is  a  case  of  more  or  less  ex- 
clusive inheritance,  or  inheritance  by  the  offspring  of 
the  whole  of  the  character  of  one  parent  and  none  of 
that  of  the  other.  Obviously,  therefore,  for  such  in- 
heritance the  law  of  ancestral  heredity  does  not  at  first 
sight  appear  to  hold.  Supposing  the  offspring  are 
equally  likely  to  take  after  one  parent  or  the  other, 
then  the  coefficient  of  regression  between  parent  and 
offspring  will  be  .5,  instead  of  .3,  as  in  the  case  of 
blended  inheritance :  between  grandparent  and  offspring 
it  will  be  .25,  instead  of  .15,  and  so  on.  Nevertheless 
it  is  probable  that  the  law  of  ancestral  heredity  is  just 
as  true  for  one  form  of  inheritance  as  for  the  other, 
only  from  the  mere  fact  of  the  inheritance  being  ex- 
clusive, it  does  not  reveal  itself  in  the  same  way.  Sup- 
posing there  is  no  alternative  between,  for  instance, 
*  "  Natural  Inheritance,"  p.  139. 


BLASTOGENIC  VARIATIONS.  133 

light  and  dark  eye-colour,  or  in  animals,  light  and  dark 
coat  colour,  then  we  can  imagine  the  various  light  and 
dark  heritages  from  each  of  the  parents,  grandparents, 
and  more  remote  ancestors  to  be  summated  and  bal- 
anced against  each  other  in  each  individual,  and,  which- 
ever reach  the  higher  figure,  be  it  by  ever  so  little  an 
amount,  to  be  thereby  enabled  to  originate  exclusively 
the  character  to  which  they  correspond.  The  constitu- 
tion of  the  germ-plasm  of  a  light  or  dark-coloured  ani- 
mal cannot  be  inferred,  therefore,  unless  the  colour  of 
its  ancestors  be  known,  for  it  may  contain  anything 
from  just  over  half  right  up  to  the  full  number  possible 
of  "  light  "  or  "  dark  "  determinants. 

That  exclusive  inheritance  obeys  the  law  of  heredity 
in  the  same  manner  as  blended  inheritance  seems  to  be 
shown  by  the  fact  that  the  striking  proof  of  the  law  re- 
ferred to  above  was  obtained  by  Galton  for  an  almost 
exclusively  inherited  character,  viz.,  coat  colour  in  Bas- 
set hounds.  Galton  believed  also  that  his  data  for 
eye-colour  in  man  afforded  considerable  support  to  the 
law  in  question.  Professor  Pearson,*  however,  seems 
to  regard  exclusive  inheritance  as  distinct  from  blended 
inheritance,  and  to  look  upon  it  as  governed  by  a  Law  of 
Reversion,  and  not  by  the  law  of  ancestral  heredity. 
Arguing  from  this  law,  we  may  suppose  that  25  per 
cent,  of  the  offspring  show  the  full  character  of  either 
parent,  2¥5  per  cent,  of  them  exhibit  or  revert  to  the 
full  character  of  each  of  the  four  grandparents,  -f  f  per 
cent,  revert  to  the  full  character  of  each  of  the  eight 
great-grandparents,  and  so  on.  However,  the  whole 

*Proc.  Roy.  Soc.,  Ixvi.  p.  140;  also  "  Grammar  of  Science,"  pp. 
486-496;  also  Phil.  Trans.  Roy.  Soc.  1901,  A.  p.  79. 


134  BLASTOGENIC  VARIATIONS. 

question  is  fraught  with  doubt  and  difficulty,  and 
greatly  lacks  the  experimental  data  necessary  for  put- 
ting it  to  an  adequate  practical  test.  Hence,  for  the 
present,  it  is  best  to  regard  the  matter  as  still  sub 
judice. 

The  law  of  heredity  and  regression  which  we  may 
consider  to  have  been  substantiated  for  sweet  peas,  for 
Basset  hounds,  and  for  man,  may  justifiably  be  ex- 
tended to  other  organisms  as  well.  It  seems  probable 
that  it  is,  in  fact,  to  use  Mr.  Galton's  words,  "  univer- 
sally applicable  to  bisexual  descent."  As  already 
stated,  however,  it  should  always  be  borne  in  mind  that 
it  deals  only  with  average  amounts,  and  not  absolute 
amounts.  Though  the  law  is  of  great  value  in  the 
breeding  of  pedigree  stock,  it  is  not  exact  enough  to 
enable  one  to  predict  with  any  accuracy  the  characters 
of  the  unborn  offspring  of  known  parents,  or  even  of 
known  grandparents  as  well  as  parents.  Nevertheless 
the  mere  fact  of  such  a  law  of  average  inheritance 
being  demonstrable,  indicates  triumphantly  how  fun- 
damentally important  is  the  constitution  of  the  germ- 
plasm  in  the  determination  of  variations. 

We  see,  then,  that  Weismann's  conclusions  as  to  the 
chief  factors  concerned  in  the  origin  of  blastogenic 
variations  are  in  the  main  confirmed,  so  far  as  it  is  pos- 
sible to  put  them  to  an  experimental  test.  It  is 
of  course  impossible  to  obtain  experimental  proof  of 
the  actual  existence  of  biophors,  determinants,  and  ids 
in  the  germ-plasm,  but  it  is  scarcely  possible  to  account 
for  the  facts  of  heredity  without  making  some  such 
hypothesis.  The  Law  of  Ancestral  Heredity  proves 
that  all  ancestors,  however  remote,  are  able  to  leave 


BLASTOGENIC  VARIATIONS.  135 

the  impress  of  their  individuality  upon  the  sex-cells,  in 
diminishing  proportion  according  to  their  remoteness. 
Such  a  fact  can  only  be  accounted  for  by  assuming  the 
existence,  in  the  germ-plasm,  of  definite  units  carrying 
definite  characters,  and  the  regular  halving  in  the  aver- 
age strength  or  amount  of  such  characters  during  the 
reducing  division  of  the  nuclear  matter  of  the  sex-cells 
which  precedes  each  act  of  sexual  reproduction. 

It  was  stated  above  that  Professor  Pearson  calcu- 
lated the  correlation  constant  between  brothers  to  be 
.4.  In  a  remarkable  memoir  recently  published  Pro- 
fessor Pearson  *  and  his  collaborators  have  collected 
together  all  the  statistics  at  present  available,  as  to  fra- 
ternal correlation  in  the  horse,  the  dog,  and  in  daphnia, 
as  well  as  in  man,  and  have  found  the  mean  of  the  con- 
stants deduced  from  19  series  of  observations  to  be 
.4479.  Individual  constants  range  from  .6934  down  to 
.1973,  but  doubtless  some  of  the  extreme  values  in 
either  direction  are,  for  various  reasons,  invalid.  It 
seems  probable,  therefore,  that  fraternal  correlation, 
whether  it  concerns  stature,  cephalic  index,  eye-colour, 
or  longevity  in  man,  or  coat  colour  in  the  dog  and  horse, 
may  be  taken  to  fluctuate  about  a  mean  value  of  .4 
to  .5. 

The  greater  part  of  this  memoir,  however,  concerns 
correlation  in  the  vegetable  kingdom.  Professor  Pear- 
son points  out  that  the  individual  puts  forth  a  number 
of  like  organs,  such  as  blood  corpuscles,  spermatozoa, 
petals  of  the  flower,  leaves  of  the  trees,  which  are  un- 
differentiated,  but  that  nevertheless  there  is  a  consider- 
able amount  of  variation  among  these  "  undifferenti- 
*  Phil.  Trans.  1901,  A.  p.  285. 


136  BLASTOGENIC  VARIATIONS. 

ated  like  organs/7  or  "  homotypes."  It  is  found,  how- 
ever, that  the  variability  of  these  like  organs  in  an  in- 
dividual is  less  than  that  of  similar  like  organs  in  all  the 
members  of  a  race  (it  being  as  a  rule  80  to  90  per  cent, 
as  great),  and  that  therefore  there  is  a  considerable  cor- 
relation between  them.  The  principle  that  like  organs 
are  correlated,  or  that  the  undiiferentiated  like  organs 
of  individuals  have  a  certain  degree  of  resemblance, 
Professor  Pearson  speaks  of  as  Jiomotyposis. 

Professor  Pearson  and  his  collaborators  have  deter- 
mined the  degree  of  homotyposis  in  22  distinct  series, 
and  have  determined,  for  instance,  the  numbers  of  leaf- 
lets on  the  leaf  of  the  Ash  (26  leaves  being  taken  from 
each  of  329  trees),  the  number  of  veins  in  the  leaf  of 
the  Spanish  Chestnut  (26  leaves  from  204  trees),  and  in 
that  of  the  Beech  (26  leaves  from  100  trees),  the 
prickles  on  the  leaf  of  the  Holly  (26  leaves  from  299 
trees),  the  stigmatic  bands  on  the  seed  capsules  of 
poppies  (10,435  capsules,  taken  from  1064  plants),  the 
sori  on  8  to  12  fronds  of  each  of  101  Hartstongue 
ferns,  etc.  The  mean  correlation  for  all  the  22  series 
was  .4570,  or  practically  the  same  value  as  was  obtained 
for  fraternal  correlation.  The  extreme  values  ranged 
from  .6311  to  .1733,  but  there  are  numerous  causes 
which  will  account,  at  least  in  part,  for  these  wide  de- 
viations from  the  average.  Supposing  that  any  of  the 
organs  measured  had  undergone  a  certain  amount  of 
differentiation  or  splitting  up  in  various  directions  (and 
this,  it  must  be  remembered,  is  always  possible,  as  there 
is  no  real  criterion  as  to  whether  any  given  organ  is 
really  undiiferentiated,  or  differentiated),  this  would 
generally  result  in  a  great  reduction  in  the  correlation; 


BLASTOGENIC  VAEIATIONS.  137 

whilst  a  heterogeneity  of  material,  such  as  a  mixture 
of  two  different  local  races,  would  tend,  as  a  rule,  to 
raise  correlation.*  Also  the  environmental  factor,  and 
the  difficulty  of  ensuring  that  all  individuals  are  of  the 
same  age,  or  in  the  same  state  of  development,  must  be 
borne  in  mind. 

Professor  Pearson  therefore  considers  that  he  is 
justified  in  assuming  that  the  intensity  of  pure  homo- 
typosis  throughout  the  vegetable  kingdom  probably 
lies  between  A  and  .5,  and  as  this  is  the  mean  value  for 
fraternal  correlation,  he  believes  that  "  heredity  is 
really  only  a  phase  of  the  wider  factor  of  homotyposis." 

In  a  criticism  of  Pearson's  conclusions,  Bateson  t 
draws  attention  to  the  fact  that  it  is  difficult  or  impos- 
sible to  distinguish  between  chance  variation  occurring 
between  members  of  a  series,  and  actual  differentiation, 
which  may  be  present  in  greater  or  less  degree.  He 
therefore  considers  that  the  average  value  of  the  homo- 
typosis coefficient  has  no  significance.  However,  Pear- 
son states  that  the  "  diversity  due  to  differentiation.  .  . 
is  the  result  of  dominating  factors  which  can  be  isolated 
and  described,"  though  he  does  not  attempt  this  in 
detail  in  his  present  memoir.  To  what  extent  he  will 
be  able  to  accomplish  it,  and  so  ultimately  obtain  the 
true  correlation  constants  of  absolutely  undifferentiated 
like  organs,  remains  to  be  seen.  J 

*L.  c.,  p.  292. 

fProc.  Roy.  Soc.,  Ixix.  p.  193. 

J  See  also  rejoinder  by  Professor  Pearson  in   Biometrika,   i.  p. 
320,  1902. 


CHAPTER  V. 
BLASTOGENIC    VARIATIONS     (Continued). 

Reversion;  commonest  in  crossed  races,  as  of  the  pigeon  and  fowl;  its 
theoretical  explanation — Prepotency;  in  the  trotting  horse  and  in 
man;  probably  due  in  large  part  to  inbreeding— Mendel's  Law  of 
Hybridisation,  and  its  range— Natural  and  artificial  plant  hybrids — 
Animal  hybrids — Sports;  probably  of  different  origin  to  normal 
variations — Artificial  production  of  monsters — Telegony;  probably 
non-existent — Parthenogenesis  in  an  Ostracod  and  in  Daphnia — 
Does  sexual  reproduction  induce  variability? — Relation  of  varia- 
bility of  individual  to  variability  of  race — Asexual  reproduction  in 
plants— Bud-variation. 

IN  the  last  chapter  we  saw  that  the  average  char- 
acters of  offspring  are  inherited  from  their  ancestors  in 
accordance  with  a  simple  and  definite  law,  but  it  re- 
mains for  us  to  discuss  several  phenomena  related  to 
this  law,  some  of  which,  indeed,  appear  to  afford  a  par- 
tial contradiction  of  it.  These  are  the  phenomena  of 
reversion,  prepotency,  the  appearance  of  sports,  and 
certain  cases  of  hybridism.  The  variations  which  show 
themselves  in  connection  with  such  phenomena,  though 
doubtless  of  less  importance  than  those  already  dis- 
cussed, are  nevertheless  in  some  instances  considerable, 
and  of  not  infrequent  occurrence.  They  therefore 
merit  a  fairly  full  discussion.  It  is  impossible,  how- 
ever, to  illustrate  this  with  many  exact  numerical  data, 
simply  because  these  do  not  exist.  One  must  as  a  rule 
remain  content  to  quote  the  descriptive  evidence  of 

138 


BLASTOGENIC  VAEIATIONS.  139 

breeders  and  others,  who  seldom  troubled  to  substan- 
tiate their  views  by  measurements  and  figures. 

The  phenomenon  of  Reversion  or  Atavism  has  long 
been  recognised,  not  only  by  agriculturalists  and 
breeders,  but  also  by  others  who  have  witnessed  its  oc- 
currence in  members  of  the  human  race.  One  of  the 
simplest  instances  of  reversion  is  that  of  a  child  or  a 
lower  animal  resembling  a  grandparent  more  closely 
than  its  immediate  parents.  Much  more  remarkable, 
however,  are  those  instances  in  which  the  resemblance 
is  to  a  remote  ancestor,  or  to  some  distant  member  in  a 
collateral  line  (supposing,  of  course,  that  these  be  held 
to  be  properly  substantiated).  Cases  of  reversion  are 
very  frequent  in  respect  of  secondary  sexual  characters, 
as  when  a  son  resembles  his  maternal  grandsire  more 
closely  than  his  paternal  in  some  such  attribute  as  a 
peculiarity  of  the  beard,  in  the  case  of  man;  of  the 
horns,  in  the  case  of  the  bull;  and  of  the  hackles  or 
comb  in  the  cock.  Also  it  is  well  known  that  certain 
diseases,  such  as  haemophilia  and  colour-blindness,  are 
frequently  transmitted  to  male  offspring  through  a 
woman  who  herself  remains  unaffected. 

For  most  of  our  knowledge  on  the  subject  of  rever- 
sion we  are  indebted  to  the  labours  of,  Charles  Darwin, 
who  obtained  most  valuable  experimental  evidence  him- 
self, besides  collecting  from  most  varied  sources  the  re- 
sults obtained  by  others.  One  of  the  most  striking  in- 
stances he  records  is  that  of  a  pointer  bitch,*  which  pro- 
duced seven  puppies.  Four  of  these  were  marked  with 
blue  and  white,  which  is  so  unusual  a  colour  with 
pointers  that  the  animal  was  thought  to  have  played 
*  "Animals  and  Plants,"  ii.  p.  8. 


140  BLASTOGENIC  VARIATIONS, 

false  with  the  greyhounds,  and  all  but  one  of  the  litter 
were  destroyed.  Two  years  later,  this  young  dog  was 
seen  by  a  friend  of  the  owner,  and  he  declared  him  to 
be  the  image  of  his  old  pointer  bitch,  the  only  blue  and 
white  pointer  of  pure  descent  he  had  ever  seen.  On 
close  enquiry,  it  was  proved  that  the  dog  was  the  great- 
great-  grandson  of  the  bitch,  and  so,  on  Galton's  law,  it 
should  have  received  only  ¥fg-  part  of  its  heritage  from 
her.  Another  even  more  remarkable  instance  is  that 
of  a  calf  which  was  coloured  in  a  very  peculiar  manner, 
its  legs,  belly,  and  part  of  the  tail  being  white,  and  the 
remainder  black.  Its  great-great-great-great-grand- 
father was  coloured  in  the  same  peculiar  manner,  but 
all  the  intermediate  offspring  were  black.  Hence  the 
calf  reverted  in  its  colour  markings  to  an  ancestor  from 
which  it  should  have  drawn  only  ^^  part  of  its 
heritage. 

It  is  when  two  distinct  races  are  crossed  that  the 
tendency  in  the  offspring  to  reversion  most  often  de- 
clares itself.  No  examples  are  more  striking  than 
those  obtained  by  Darwin  in  the  case  of  the  domestic 
pigeon.  For  instance,*  he  paired  a  mongrel  female 
Barb-fantail  with  a  mongrel  male  Barb-spot,  neither  of 
these  mongrels  having  the  least  blue  about  them. 
"  Nevertheless  the  offspring  from  these  two  mongrels 
was  of  exactly  the  same  blue  tint  as  that  of  the  wild 
rock-pigeon  from  the  Shetland  Islands  over  the  whole 
back  and  wings;  the  double  black  wing  bars  were 
equally  conspicuous;  the  tail  was  exactly  alike  in  all  its 
characters,  and  the  croup  was  pure  white;  the  head, 
however,  was  tinted  with  a  shade  of  red,  evidently  de- 
*L.  c.t  i.  p.  209. 


BLASTOGENIC  VARIATIONS.  141 

rived  from  the  Spot,  and  was  of  a  paler  blue  than  in  the 
rock-pigeon,  as  was  the  stomach.  So  that  two  black 
Barbs,  a  red  Spot,  and  a  white  Fantail,  as  the  four 
purely-bred  grandparents,  produced  a  bird  exhibiting 
the  general  blue  colour,  together  with  every  character- 
istic mark,  the  wild  Columba  livia" 

Professor  J.  C.  Ewart,  in  the  breeding  experiments 
he  has  recently  beep  carrying  out  at  Penycuik,*  has  ob- 
tained an  equally  striking  case  of  reversion  in  the  case 
of  the  pigeon.  He  crossed  a  pure  white  Fantail  cock 
with  the  offspring  of  an  Owl  and  an  Archangel.  One 
of  the  young  of  this  complex  pair  had  the  colouration 
of  the  Shetland  rock-pigeon,  whilst-  the  other  resembled 
the  Indian  rock-pigeon  in  having  a  blue  croup  and  the 
front  part  of  the  wings  chequered.  In  this  second  bird 
there  was  complete  reversion  as  to  colour,  and  in  the 
first,  wherever  measurements  were  possible,  there  was 
practically  complete  reversion  also  as  to  form.  The 
tail  feathers  were  twelve  in  number  and  showed  but 
the  faintest  indications  of  any  colour  inheritance  from 
their  immediate  parents.  An  additional  point  of 
interest  lay  in  the  fact  that  in  disposition  the  bird 
seemed  wilder  and  more  shy  than  the  domestic  breeds 
usually  are. 

Many  other  instances  might  be  quoted  from  Darwin 
and  others  to  prove  that  this  tendency  to  the  production 
of  offspring  of  a  blue  colour,  with  the  same  charac- 
teristic marks  as  Columba  livia,  is  present  in  all  the 
chief  domestic  races  of  pigeon.  It  shows  itself  more 
especially  when  these  domestic  races  are  crossed,  but 
may  even  appear  occasionally  in  the  purely  bred  races, 
*  "  The  Penycuik  Experiments,"  Edinburgh,  1899. 


142  BLASTOGENIC  VARIATIONS. 

Similar  phenomena  show  themselves  in  other  domestic 
animals  besides  the  pigeon,  though  they  are  seldom  so 
striking  or  so  clear.  Thus  in  some  cases  the  wild  an- 
cestor or  ancestors  are  quite  unknown,  and  hence  one  is 
debarred  from  coming  to  any  certain  conclusions  as  to 
whether  reversion  is  present  or  not.  The  Game  fowl, 
however,  and  probably  most  other  domestic  breeds 
of  fowl,  may  with  considerable  confidence  be  de- 
rived from  the  jungle  fowl,  Gallus  lankiva.  Now 
purely  bred  Game,  Malay,  Cochin,  Dorking,  Bantam, 
and  Silk  fowls  may  frequently  or  occasionally  be  met 
with,  which  are  almost  identical  in  plumage  with  the 
wild  Gallus  bankiva.  The  most  striking  instance  ob- 
tained by  Darwin  *  is  one  in  which  a  glossy  green-black 
Spanish  cock  was  crossed  with  a  diminutive  white  Silk 
hen.  Both  of  these  breeds  are  ancient,  and  have  long 
been  known  to  breed  true.  All  the  offspring  from  this 
cross  were  coal  black,  and  all  plainly  showed  their 
parentage  in  having  blackish  combs  and  bones;  but  as 
the  young  cocks  grew,  one  became  a  gorgeous  bird, 
closely  resembling  the  wild  G.  bankiva,  but  with  the  red 
feathers  rather  darker.  In  all  but  a  few  details  there 
was  the  closest  resemblance. 

In  recent  years  a  series  of  interesting  observations 
has  been  carried  out  by  von  Guiata  upon  mice.f 
Fifty-five  Japanese  waltzing  mice  were  crossed  with 
white  mice  belonging  to  a  race  bred  by  Weismann  for 
eight  years,  and  these  crosses  were  continued  through 

*L.  c.  ii.  p.  253. 

fBer.  Naturf.  Ges.  zu  Freiburg,  Bd.  x.  p.  317,  1898,  and  Bd.  xi. 
p.  131,  1900.  Review  by  Davenport  in  Biol.  Bulletin,  ii.  p.  121, 
1901. 


BLASTOGENIC  VARIATIONS.  143 

seven  generations.  Japanese  waltzing  mice  are  mostly 
black  and  white,  i.  e.,  piebald,  in  colour,  but  their 
crosses  and  reciprocal  crosses  with  the  albino  race 
yielded  a  most  unexpected  result.  The  whole  of  the 
offspring  produced  were  of  a  gray  colour,  indistin- 
guishable in  respect  either  of  colour  or  of  size  from  the 
common  house  mouse.  The  waltzing  action  was  en- 
tirely wanting,  the  reversion  being  apparently  com- 
plete. Heacke  had  obtained  a  similar  result  on  cross- 
ing the  same  races.*  In  the  third  generation,  how- 
ever, the  type  was  broken,  for  the  44  young  produced 
by  the  mating  of  4  pairs  of  the  reverting  gray  mice 
consisted  of  8  waltzers  (albino,  spotted,  gray,  and 
black),  11  pure  albinos,  and  25  gray  mice.  In  the  sub- 
sequent generations,  the  albinos  and  also  the  gray  and 
the  spotted  mice  were  found  to  breed  true.  Gray  mice 
crossed  with  white  yielded  mostly  gray  offspring,  but  a 
certain  number  of  waltzers. 

Of  the  occurrence  of  reversion  there  can  thus  be  no 
question.  In  fact,  its  appearance  in  the  offspring  of 
crossed  races  is  by  no  means  an  infrequent  phenomenon. 
The  reversion  of  hybrids  and  mongrels  to  one  of  their 
pure  parent  forms,  after  an  interval  of  two  or  more 
generations,  is  especially  common.  Hence  it  would 
seem  that  the  act  of  crossing  in  itself  gives  an  impulse 
towards  reversion.  Why  and  how  this  is  the  case  must 
be  more  or  less  a  matter  of  conjecture.  Indeed,  this  is 
equally  true  for  all  the  phenomena  of  reversion,  but  I 
think  that  a  brief  consideration  of  certain  presumptions 
regarding  the  germ-plasm  as  the  bearer  of  hereditary- 
characters  will  show  that,  after  all,  we  are  not  dealing 
*Biol.  Central.,  Bd.  xv.  p.  44, 1895. 


144  BLASTOGENIC  VARIATIONS. 

with  anything  more  mysterious  and  remarkable  than  is 
found  in  most  of  the  phenomena  of  nature.  Thus  tak- 
ing it  for  granted  that  each  of  the  parts  of  an  organism 
capable  of  independent  variation  from  the  germ  on- 
wards has  a  definite  representative  or  determinant  of 
some  sort  in  the  germ-plasm,  what  proportion  does  the 
mass  of  the  determinants  of,  say,  all  the  characters 
which  distinguish  a  pouter  or  a  fantail  pigeon  from  a 
rock  pigeon,  bear  to  the  mass  of  the  determinants  which 
represent  the  species  pigeon,  as  such?  Let  us  suppose 
that  the  average  total  differences  between  the  char- 
acters of  species  of  the  same  genus  be  counted  as  one 
unit,  what  would  be  the  number  of  units  corresponding 
to  differences  between  the  characters  of  genera,  fam- 
ilies, orders,  and  so  on?  No  two  biologists  would  judge 
alike,  and  of  course  it  is  impossible  to  estimate  them; 
but,  for  the  sake  of  our  argument,  let  us  attempt  some 
sort  of  rough  numerical  estimate  as  to  what  these  dif- 
ferences might  be.  Let  us  assume  that,  if  species  on 
an  average  differ  by  one  unit  in  the  sum  total  of  char- 
acters, genera  differ  by  three  units,  and  families  by  per- 
haps ten  units.  Orders  might  differ  by  25  units,  classes 
by  50  units,  and  phyla  by  100  units.  Therefore  we 
assume  that  an  individual  of  one  phylum,  in  the  sum 
total  of  its  characters,  is  100  times  more  different  from 
an  individual  of  another  phylum  than  is  one  species 
from  another  of  the  same  genus.  The  difference  be- 
tween the  highest  Vertebrate  and  the  lowest  Protophyte 
would  probably  be  considered  to  be  perhaps  ten  times 
greater  than  this,  but  let  that  pass.  Let  us  take  it  that 
the  sum  total  of  characters  represented  by  any  species  of 
pigeon  is  100  units,  of  which  the  total  characters  pe- 


BLASTOGENIC  VARIATIONS.  145 

culiar  to  a  rock  or  other  pigeon,  as  such,  is  one  unit. 
Also  let  it  be  granted  that  the  characters  separating  any 
variety  of  the  pigeon  from  the  ancestral  rock  pigeon  are 
of  the  same  value  as  those  separating  species  of  the  same 
genus,  namely,  one  unit.  Now  in  the  gradual  course  of 
evolution  of  a  domestic  variety  of  pigeon  from  a  rock 
pigeon,  we  may  assume  that  the  total  amount  of  germ- 
plasm  bearing  hereditary  characters  has  remained  prac- 
tically constant,  and  hence,  as  one  unit  of  determinants 
has  been  added  on  to  the  rock  pigeon  germ-plasm,  one 
must  have  disappeared.  Now  did  this  unit  of  deter- 
minants corresponding  to  the  characters  of  the  domes- 
tic variety  of  pigeon  replace  that  of  the  rock  pigeon,  or 
was  it  superimposed  on  it  ?  Embryology  seems  to  teach 
us  that  once  any  character  is,  as  it  were,  laid  down  in 
the  germ-plasm,  it  is  fixed  there,  and  as  a  rule  only  very 
slowly  dwindles  away  by  a  process  of  gradual  dilution 
by  subsequent  ontogenetic  stages.  It  seems  reasonable 
to  assume,  therefore,  that  the  determinants  are  re- 
placed in  proportion  to  the  relative  amounts  of  them 
present,  or  that,  on  an  average,  -ffo  of  the  replaced 
unit  concern  the  sum  total  of  hereditary  characters 
which  go  to  constitute  the  species  pigeon,  and  y^ 
those  peculiar  to  the  species  blue  rock  pigeon.  The 
germ-plasm  of  a  domestic  pigeon  will  therefore  be  made 
up  of  1  unit  of  determinants  corresponding  to  the  char- 
acters domestic  pigeon,  .99  of  a  unit  corresponding  to 
the  characters  Hue  rock  pigeon,  and  98.01  units  cor- 
responding to  the  characters  species  pigeon.  It  there- 
fore follows  that  the  hereditary  characters  of  the  an- 
cestral rock  pigeon  are  almost  as  strongly  represented 
in  the  germ-plasm  of  a  domestic  pigeon  as  they  were 


146  BLASTOGENIC  VARIATIONS. 

originally,  only  that  they  seldom  have  an  opportunity  of 
showing  themselves.  They  are  covered  up  by  the  more 
recently  acquired  characters,  and  it  is  only  under  ex- 
ceptional circumstances  that  they  are  able  to  reveal 
themselves.  When,  for  instance,  two  distinct  races  of 
pigeon,  such  as  a  pouter  and  a  f  antail,  are  crossed,  then 
the  offspring  would  on  an  average  receive  .5  of  a  unit 
of  determinants  corresponding  to  each  of  the  special 
group  of  characters  pouter  and  f  antail,  the  same  .99  of 
a  unit  corresponding  to  the  characters  blue  rock  pigeon, 
and  98.01  units  corresponding  to  the  characters  species 
pigeon.  If,  then,  the  determinants  of  pouter  and  fan- 
tail  do  not  to  any  great  extent  correspond,  what  wonder 
is  it  that  they  more  or  less  neutralise  each  other,  and 
allow  the  blue  rock  pigeon  determinants  to  gain  the 
upper  hand,  and  show  their  presence  ? 

This  view  of  the  constitution  of  the  germ-plasm  may 
at  first  sight  seem  contrary  to  the  law  of  ancestral 
heredity,  but  in  reality  it  is  not  so.  A  man  may  receive 
a  quarter  of  his  hereditary  characters  from  each  parent, 
and  a  sixteenth  from  each  grandparent,  but  all  except 
a  very  minute  proportion  of  these  characters  are  com- 
mon to  all  men,  they  being,  in  fact,  the  characters 
proper  to  the  species  Homo  sapiens,  as  such.  Instead 
of  a  quarter  of  a  unit  from  each  parent,  a  man  in  reality 
receives  only  a  hundredth  or  a  thousandth  of  a  unit  of 
characters  peculiar  to  the  parent  as  such,  all  the  rest 
being  the  characters  common  to  all  members  of  the 
race.  Even  this  minute  fraction  of  a  unit  does  not  in 
any  way  represent  characters  acquired  by  the  parent 
during  his  life-time,  but  is  itself  built  up  of  proportions 
of  peculiar  characters  received  from  his  parents,  grand- 


BLASTOGENIC  VARIATIONS.  147 

parents  and  other  ancestors  in  accordance  with  the  law 
of  heredity. 

It  seems,  then,  that  the  sudden  reappearance  of  an- 
cestral characters  ought  not  to  be  regarded  as  a  very 
remarkable  phenomenon,  but  certain  other  cases  of  re- 
version offer  a  greater  difficulty.  Thus  cases  such  as 
that  above  mentioned  of  a  calf  reverting  to  the  colour 
marking  of  an  ancestor  six  generations  back,  if  of  at  all 
frequent  occurrence,  are  truly  remarkable.  If  of  only 
very  infrequent  occurrence,  however,  they  may  perhaps 
be  ascribed  to  a  mere  coincidence,  or  to  like  conditions 
of  environment  having  acted  on  both  ancestor  and  de- 
scendant, and  produced  like  results. 

Prepotency.  The  phenomenon  of  prepotency  of  cer- 
tain individuals,  races,  and  species  in  the  transmission 
of  their  characters  is  a  very  common  one,  and  it  merits 
our  consideration,  in  that  it  is  an  important  factor  in 
the  production  of  variations.  As  a  rule,  the  offspring 
of  dissimilar  parents  are  in  most  respects  of  an  inter- 
mediate character.  Frequently,  however,  they  more  or 
less  closely  resemble  one  parent  in  one  part,  and  the 
other  parent  in  another  part.  Less  seldom  they  show 
a  much  closer  resemblance  to  one  parent  than  to  the 
other,  or  may  apparently  resemble  one  parent  in  every 
respect,  to  the  entire  exclusion  of  the  other  parent. 
We  may  here  be  dealing  with  true  cases  of  prepotency, 
or  it  may  be  that  the  characters  in  question  are  for 
some  unknown  reason  unable  to  blend,  and  so  be  neces- 
sarily transmissible  only  in  toto  from  one  parent  to  the 
other.  For  instance,  it  is  well  known  that  certain  do- 
mestic animals,  such  as  the  cat,  show  only  a  few  well- 
defined  differences  of  colour  marking,  such  as  white, 


148  BLASTOQENIC   VARIATIONS. 

black,  tabby,  and  tortoise-shell,  and  though  they  breed 
promiscuously,  very  seldom  throw  intermediate  colours. 
Again,  Sir  R.  Heron  crossed  during  many  years  white, 
black,  brown,  and  fawn-coloured  rabbits,  and  never  once 
got  these  colours  mingled  in  the  same  animal,  but  often 
got  all  four  colours  in  the  same  litter.*  All  the  off- 
spring of  dissimilarly  coloured  parents  may  therefore 
resemble  either  parent,  or  some  resemble  one  and 
others  the  other,  possibly  quite  apart  from  any  ques- 
tion of  prepotency. 

Of  undoubted  cases  of  prepotency,  but  few  have  been 
recorded  with  much  detail  or  exactness.  Of  those  col- 
lected by  Darwin,  the  most  striking  is  that  of  a  famous 
black  grayhound,f  which  "  invariably  got  all  his  pup- 
pies black,  no  matter  what  was  the  colour  of  the  bitch  " ; 
but  this  dog  "  had  a  preponderance  of  black  in  his  blood 
both  on  the  sire  and  dam  side,"  a  point  which  will  be 
referred  to  again  later.  Again,  the  famous  bull  Fav- 
ourite is  believed  to  have  had  a  prepotent  influence  on 
the  shorthorn  race.  The  male  Manx  cat  appears  to  be 
prepotent  in  transmitting  his  tailless  condition.  Pro- 
fessor EwartJ  has  recorded  a  few  additional  cases  of 
prepotency.  Thus  a  well-known  breeder  of  highly  bred 
ponies  used  to  boast  that  he  had  a  filly  so  prepotent 
through  inbreeding  that,  though  she  were  sent  to  the 
best  Clydesdale  stallion  in  Scotland,  she  would  throw  a 
colt  showing  no  cart-horse  blood,  provided  always  that 
the  Clydesdale  was  not  also  the  product  of  inbreeding. 
Again,  Professor  Ewart  points  out  that  Jews,  as  a  race, 

*  "  Animals  and  Plants,"  ii,  p.  70. 

\L.  c.,ii.  p.  40. 

\  "  Penycuik  Experiments/'  p.  xli. 


BLASTOGENIC  VARIATIONS. 


149 


are  strongly  prepotent,  probably  because  they  are  purer 
bred  than  other  races. 

A  numerical  estimate  of  the  frequency  with  which 
different  grades  of  prepotency  are  distributed  appears 
to  have  been  attempted  for  the  first  time  quite  recently 
by  Mr.  Galton.*  From  data  given  in  Wallace's  Year 
Book  of  American  Trotting  Horses,  he  has  determined 
the  numbers  of  offspring  of  a  certain  standard,  pro- 
duced by  various  sires  and  dams.  A  standard  per- 
former is  a  horse  which  has  succeeded  in  trotting  a  mile 
in  2  min.  30  seconds  or  less,  or  in  pacing  (ambling)  a 
mile  in  2  min.  25  seconds  or  less.  Data  concerning  the 
offspring  of  716  sires  and  494  dams  were  available,  and 
the  following  were  the  percentage  proportions  of 
"  standard  performers  "  produced  by  them. 


NUMBER  OF  STANDARD  PERFORMERS  PRODUCED  BT  A 

SINGLE  PARENT,  SIRE  OR  DAM. 

1 

2 

3 

4 

5 

6  to 

11  to 

21  to 

31  to 

41  to 

51  and 

Total 
Parents 

10 

20 

30 

40 

50 

above 

Sires 

46 

17 

10 

7 

3 

9 

4 

1 

1 

1 

1 

100 

Dams 

50 

35 

10 

3 

1 

1 

- 

- 

- 

- 

- 

100 

This  table  would  seem  to  show  that  the  prepotency  of 
certain  sires  is  enormous,  even  allowing  for  the  tend- 
ency of  breeders  to  send  the  best  mares  to  the  best 
horses.  Thus  the  horse  Happy  Medium  had  92  dis- 
tinguished offspring,  and  Electioneer  no  less  than  154. 
The  same  results  are  indicated  by  the  produce  of  the 
dams,  though  the  figures  are  less  striking  owing  to  the 
relative  fewness  of  their  offspring.  A  sire  produces 
*  Nature,  vol.  Iviii.  p.  246,  1898. 


150  BLASTOGENIC   VARIATIONS. 

30  foals  annually,  but  a  dam  only  one,  hence  the  pro- 
duction of  respectively  7,  8,  and  9  standard  performers 
by  three  mares  is  very  remarkable.  Professor  Pear- 
son,* however,  does  not  accept  the  high  degree  of  pre- 
potency which  these  figures  seem  to  indicate.  He 
points  out  that  the  fact  of  certain  sires  producing  such  a 
preponderance  of  standard  performers  is  largely  due  to 
their  exceptional  pedigrees.  It  is  also  due  to  the 
second-rate  stallions  being  given  far  less  chance  of  pro- 
ducing performers,  in  that  the  mares  sent  them  are 
often  inferior,  or  past  their  most  intense  fecundity,  as 
well  as  being  fewer  in  number. 

In  discussing  the  law  of  heredity  in  the  last  chapter, 
it  was  tacitly  assumed  that  the  heritage  from  each 
parent  was  the  same,  or  that  both  parents  were  equi- 
potent.  This  does  not  seem  to  be  necessarily  the  case, 
however,  as  Professor  Pearson  finds  that  in  man  the 
father  is  slightly  prepotent  over  the  mother  for  the  off- 
spring of  both  sexes,  f  Thus  a  determination  of  the 
coefficient  of  correlation  in  respect  of  stature  and  of 
head  index,  gave  the  following  figures: 

Coeff.  of  correlation 

Father  and  son  (Middle  class  English)  .396  (for  stature) 

"    daughter        "         "  "  .360    " 

Mother  and  son  "         "  "  .302    " 

"      daughter      "         "  "  .284    " 

"        "      son  (N.  American  Indians)  .370  (for  head  index) 

"      daughter          "  "  .300    "      " 

The  average  correlation  between  stature  of  father 
and  of  offspring  was  thus  .378,  and  between  that  of 
mother  and  of  offspring  .293,  or  22.5  per  cent.  less.  The 

*  Nature,  vol.  Iviii.  p.  292. 

f  "  Grammar  of  Science,"  p.  458. 


BLASTOGENIC  VARIATIONS.  151 

average  correlation  between  parent  and  offspring  was 
thus  .335,  instead  of  the  theoretical  .3.  Pearson  thinks 
this  high  value  may  be  due  to  assortive  mating.*  The 
number  of  data  available  for  calculating  these  constants 
was  not  very  great,  so  that  they  cannot  be  accepted 
as  final,  but  there  seems  little  doubt  of  the  existence  of 
a  small  degree  of  male  prepotency.  It  should  be  no- 
ticed, also,  that  the  intensity  of  the  heredity  is  stronger 
in  the  son  than  in  the  daughter,  and  this  not  only  for 
stature  in  the  English  race,  but  also  for  cephalic  index 
in  the  North  American  Indians.  A  similar  relation 
was  found  in  respect  of  eye-colour,  hence  Pearson  con- 
siders that  in  man  it  may  be  a  general  rule  for  the  male 
to  inherit  more  than  the  female. 

A  comparison  of  other  coefficients  of  correlation 
seemed  to  show  that  the  hereditary  resemblance  be- 
tween brother  and  brother,  or  sister  and  sister,  is  greater 
than  that  between  brother  and  sister.  This  was  true  for 
stature,  head  index,  and  eye-colour  in  man,  and  also  for 
coat-colour  in  thoroughbred-race  horses.  It  would 
therefore  follow  that  inheritance  in  a  line  through  one 
sex  is  prepotent  over  inheritance  with  a  change  of  sex, 
or  that,  for  instance,  a  man  would  resemble  his  paternal 
more  closely  than  his  maternal  grandfather. 

It  is  not  to  be  imagined  that  prepotency  of  the  male 
over  the  female  is  in  any  way  a  general  law.  Thus  in 
thoroughbred  horses,  sire  and  dam  are  equipotent  in  the 
transmission  of  coat-colour.  In  Basset  hounds,  on  the 
other  hand,  Galtonf  found  that  the  female  was  pre- 
potent over  the  male  in  transmitting  colour  in  about  the 

*L.  c.,p.  457. 

fProc.  Roy.  Soc.,  Ixi.  p.  404,  1897. 


152  BLASTOGENIC   VARIATIONS. 

proportion  of  6  to  5.  "Whatever  the  degree  of  pre- 
potency of  one  parent  over  another — and  with  similarly 
bred  stocks  it  is  probably  never  very  great — we  must 
conclude  that  the  average  contribution  of  both  parents 
together  still  remains  at  a  half,  that  of  grandparents  at 
a  quarter,  and  so  on.  It  is  only  the  relative  propor- 
tions contributed  by  the  two  sexes  which  differ.  Thus 
it  will  be  remembered  that  it  was  the  Basset  hound 
data  which  afforded  Galton  such  valuable  evidence  in 
support  of  his  law. 

But  what  view  are  we  to  take  of  the  more  striking 
instances  of  prepotency  mentioned  above?  Are  they 
also  conformable  to  the  law  of  heredity,  or  are  they  ab- 
normal and  exceptional?  Galton  himself  has  come  to 
the  conclusion  that  high  prepotency  does  not  arise 
through  normal  variation,  but  must  rank  as  a  highly 
heritable  sport.  As  has  been  mentioned  in  a  previous 
chapter,  there  is  no  adequate  proof  that  sports  transmit 
their  characters  more  persistently  than  other  varia- 
tions, and  in  any  case  it  is  probably  unnecessary  to  as- 
sume that  prepotency  is  other  than  a  special  case  of  the 
law  of  heredity.  Thus  we  saw  in  the  above-mentioned 
case  of  the  black  grajrhound,  that  the  dog  had  a  prepon- 
derance of  black  in  his  blood,  both  on  the  sire  and  dam 
side,  whilst  both  the  instances  of  prepotency  mentioned 
by  Professor  Ewart  seem  largely  attributable  to  in- 
breeding. This  inbreeding,  according  to  Professor 
Ewart,  induces  prepotency  by  fixing  the  characters  of 
the  particular  variety  selected.  But  what  is  really 
meant  by  fixing  a  character?  To  adequately  com- 
prehend the  meaning  of  the  term  it  is  only  neces- 
sary to  examine  Galton's  law  of  ancestral  heredity  a 


BLASTOGENIC  VARIATIONS.  15$ 

little  more  in  detail  than  we  did  in  the  last  chapter,  and 
it  will  then  be  obvious  that  a  character  becomes  more 
and  more  fixed  in  the  offspring,  the  more  and  more  fully 
it  is  represented  in  the  parents,  grandparents,  and  more 
remote  ancestors.  Thus,  according  to  the  law,  offspring 
receive  a  half  of  their  heritage  from  their  parents,  a 
quarter  from  their  grandparents,  and  so  forth.  But  of 
what  is  this  half  and  this  quarter  made  up  ?  Obviously 
half  of  the  parental  half  heritage,  or  a  quarter  in  all, 
was  received  from  their  parents,  and  a  quarter  of  a  half, 
or  an  eighth  in  all,  from  their  grandparents,  and  so  on; 
whilst,  as  regards  the  quarter  heritage  received  by  the 
offspring  from  their  grandparents,  a  half  of  it,  or  an 
eighth  in  all,  was  received  from  their  parents,  and  so 
on.  By  tracing  back  the  heritages  in  this  way,  it  is 
therefore  possible  to  calculate  the  absolute  amounts  of 
any  character  or  strain  present  in  offspring,  as  distin- 
guished from  the  relative  amounts;  relative,  that  is,  to 
those  present  in  the  parents  and  other  ancestors.  For 
instance,  supposing  the  parents  and  parents  only  had 
been  selected  in  respect  of  any  particular  character,  the 
condition  of  the  previous  ancestors  being  entirely  un- 
known, then,  as  we  have  already  seen,  the  offspring  will 
exhibit  these  exceptional  characters  in  only  .6  of  their 
full  strength,  or  will  have  regressed  to  this  extent  to- 
wards the  general  mean  of  the  race.  Supposing  the 
grandparents  have  been  selected  in  respect  of  the  same 
characters,  as  well  as  the  parents,  then  Pearson  *  has 
calculated  that  the  offspring  will  exhibit  the  characters 
in  .8049  of  their  full  strength;  if  the  great-grand- 
parents also,  then  in  .9027  of  their  strength;  and  if  still 
*Proc.  Roy.  Soc.,  Ixii,  p.  39! 


154  BLASTOGENIC  VARIATIONS. 

three  other  generations  back  be  selected,  then  in  .9879 
of  their  full  strength.  That  is  to  say,  "  after  six  gener- 
ations of  selection  the  selected  individuals  will,  without 
further  selection,  breed  true  to  the  selected  type 
within  nearly  1  per  cent,  of  its  value."  In  fact,  prac- 
tically all  regression  towards  mediocrity  will  have  been 
weeded  out.  Supposing  now,  some  variety  of  a  species 
which  had  been  bred  true  to  its  varietal  characters 
for  only  two  generations  were  crossed  with  another 
variety  of  the  same  species  which  had  been  bred  true 
to  Us  characters  for  six,  then  the  resulting  offspring 

8049 

would  receive  ' — ~ —  =  .4025  of  the  characters  of  one 
& 

9879 
parent,      0      =  .  4940  of  those  of  the  more  thorough- 

<0 

bred  parent,  and  .1035  of  unknown  blood.  Knowing 
as  we  do  that  many  characters  show  little  or  no  tend- 
ency to  blend,  it  would  not  be  very  remarkable  if  the 
offspring  resembled  the  more  thoroughbred  parent  to 
the  partial  or  almost  complete  exclusion  of  the  ill  bred. 
That  is  to  say,  the  one  parent  would  prove  itself 
strongly  prepotent,  simply  through  its  characters  hav- 
ing become  fixed  through  inbreeding. 

Hybridisation.  Though,  as  a  rule,  intercrossing  be- 
tween different  varieties  of  the  same  species  tends  to 
produce  uniformity  of  character,  yet  it  may  also  very 
frequently  lead  to  the  production  of  increased  varia- 
bility, not  only  by  the  partial  or  complete  absence  of 
blending  of  the  parental  characters,  but  also  by  the  ap- 
pearance of  seemingly  fresh  characters,  due  to  rever- 
sion or  some  other  cause. 

Our  knowledge  of  the  laws  governing  hybridisation 


BLASTOGENIC  VARIATIONS.  155 

is  chiefly  derived  from  observations  on  plants,  for  by 
reason  of  the  ease  and  success  with  which  they  are 
carried  out,  and  the  scientific  and  practical  results  ob- 
tained, these  altogether  outweigh  the  comparatively 
few  observations  which  have  been  made  on  animals. 
Though  much  of  the  evidence  obtained  is  variable  and 
contradictory,  yet  some  of  it  has  afforded  results  of 
striking  lucidity.  Especially  is  this  the  case  as  regards 
what  may  be  termed  Mendel's  Law  of  Hybridisation. 
Though  discovered  as  long  ago  as  1865,*  this  important 
generalisation  has  passed  almost  unnoticed  until  the 
last  year  or  two,  when  it  was  independently  re-discov- 
ered and  confirmed  by  De  Yries,  by  Correns,  and  by 
Tschermak.  Mendel's  observations  extended  over 
eight  years,  during  which  over  10,000  plants  were  ex- 
amined. Most  of  them  were  made  upon  the  different 
varieties  of  the  pea,  Pisum  sativum.  The  varieties 
employed  differed  in  respect  of  (1)  the  form  of  the  ripe 
seeds,  these  being  either  nearly  round,  or  angular  and 
wrinkled;  (2)  the  colour  of  the  cotyledons,  these  being 
various  shades  of  yellow  or  green;  (3)  the  colour  of  the 
seed  coat,  this  being  either  white,  gray,  or  brown;  (4) 
the  form  of  the  ripe  pods,  these  being  either  simply  in- 
flated, or  deeply  constricted  between  the  seeds;  (5)  the 
colour  of  the  unripe  pods,  this  being  yellow,  or  light  to 
dark  green;  (6)  the  position  of  the  flowers,  either  axial 
or  terminal;  (7)  the  length  of  stem. 

On  uniting  each  of  these  two  differentiating  char- 
acters by  cross-fertilisation,  the  hybrids  obtained  in 
each  case  were  found  to  resemble  only  one  of  their 

*  Abhandl.  d.  naturforsch.  Ver.  in  Brttnn,  Bd.  iv.  1865.    Trans- 
lated by  W.  Bateson  in  J.  Roy.  Horticult.  Soc.,  xxvi.  p.  1,  1901, 


156 


BLASTOGENIC   VARIATIONS. 


parent  forms,  and  to  show  little  or  no  trace  of  the 
other.  The  characters  thus  appearing  were  termed  by 
Mendel  dominant,  and  the  characters  becoming  latent 
in  the  process,  recessive.  In  the  next  generation,  how- 
ever, the  seeds  from  these  dominant  hybrids  betrayed 
their  mixed  origin,  for  instead  of  maintaining  the  pure 
dominant  character,  on  an  average  one  out  of  every 
four  of  the  plants  or  seeds  obtained  reverted  to  the  re- 
cessive parent  form.  The  following  are  the  actual 
numbers  of  plants  and  seeds  examined  by  Mendel  in 
respect  of  the  various  differentiating  characters  above 
mentioned: 

Proportion  of  Dominant 
to  Recessive. 


(1)  253hybr 
(2)  258 
(3)  925 
(4)  1187 
(5)  580 
(6)  858 
(7)  1064 

ds  (gave  7324  seeds) 
(  "  8023  "  ) 

Mean 


2.96 
3.01 
3.15 
2.95: 

2.82: 
3.14: 

2.84: 
2.98 


It  will  be  seen  that  the  proportion  of  3:1  is  fairly 
evenly  maintained  in  respect  of  all  the  characters  ob- 
served. 

The  observations  on  the  next  and  succeeding  genera- 
tions afforded  an  even  more  remarkable  result  than  this, 
for  they  proved  that  the  recessive  forms  obtained  in  the 
second  generation  were  absolutely  pure.  Thus  the 
seeds  obtained  by  crossing  them  amongst  each  other, 
or  by  self-fertilisation,  yielded  offspring  which  never 
showed  any  trace  of  the  dominant  grand-parental  char- 
acters. The  dominant  forms,  on  the  other  hand,  which 
of  course  were  self -fertilised,  underwent  a  further  split- 
ting up.  A  third  of  them  yielded  plants  which  in  sub- 


BLASTOGENIC  VAEIATIONS.  157 

sequent  generations  proved  themselves  to  be  pure  domi- 
nant forms,  whilst  two  thirds  of  them  still  retained 
their  hybrid  nature,  as  was  shown  by  their  yielding,  in 
the  next  generation,  recessive  and  dominant  forms  in 
the  proportion  of  1:3.  The  gradual  resolution  of  the 
original  hybrid  forms  into  pure  parental  forms  may  be 
represented  diagrammatically  thus,*  it  being  assumed 
that  64  hybrids  with  yellow  cotyledons  had  been  pro- 
duced by  the  crossing  of  parental  forms  having  respect- 
ively green  and  yellow  cotyledons. 

Parents    I.  Gen. 
green 


64  yellow 


yellow 

We  see  that  of  the  plants  produced  by  crossing  the 
original  64  yellow  hybrids  haphazard  amongst  them- 
selves, a  quarter  are  of  the  pure  green  form,  a  quarter 
of  the  pure  yellow  form,  and  a  half  of  them  hybrids 
with  the  yellow  character  dominating.  On  crossing 
these  hybrids  among  themselves,  we  see  that  in  each 
subsequent  generation  their  number  is  reduced  by  half, 
till  in  the  seventh  generation  only  1  of  the  original  64 
hybrids  would  be  still  remaining. 

The  explanation  of  this  result  was  clearly  laid  down 
by  Mendel,  he  supposing  that  the  cross-bred  plant  pro- 
duced pollen  grains  and  ovules,  each  of  which  bore  only 
one  of  the  alternative  varietal  characters,  and  not  both. 
If  D  and  R  represent  the  two  characters  present  in  dif- 

*  Modified  from    Correns  (Ber.  d.   deutsch.  bot.   Gesell.,  xvii. 
p.  162,  1900. 


II.  Gen. 

16  green 

r 

[32  yellow  1 
4S\                    I* 
[  16  yellow 

III.  Gen. 
16  green 

8  green          f 
(  16  yellow  j 
4  (8  yellow 
16  yellow 

IV.  Gen.        V.  Gen. 

16  green       16  green 
8  green         8  green 
4  green        4  green 
(  2  green 
{  8  yellow   •{  4  yellow  (by.) 
(  2  yellow 
4  yellow      4  yellow 
8  yellow       8  yellow 
16  yellow     16  yellow 

158  BLASTOGENIC   VARIATIONS. 

f  erent  ovules  of  the  hybrids,  and  d  and  r  those  in  pol- 
len grains,  then  on  crossing  these  hybrids  haphazard, 
the  germ  cells  giving  rise  to  the  next  generation  will 
unite  so  as  to  form  Dd  +  Dr  +  dR  -\-Rr.  Now  Men- 
del found  that  it  was  perfectly  immaterial  whether  the 
dominant  character  belonged  to  the  male  or  the  female 
plant,  and  so  it  follows  that  we  should  get  twice  as 
many  similar  hybrid  forms  (Dr  and  dR)  as  of  pure 
dominant  or  pure  recessive. 

If  parental  forms  possessing  two  or  more  differentiat- 
ing characters  be  crossed,  the  law  of  alternative  herit- 
age continues  to  hold,  though  it  necessarily  becomes 
somewhat  more  complicated.  For  instance,  Mendel 
crossed  seed  parents  with  round  seeds  (A),  and  yellow 
cotyledons  (.£?),  with  pollen  from  plants  having  angular 
seeds  (a),  and  green  cotyledons  (6).  The  hybrids 
would  therefore  consist  of  plants  with  germ  cells  hav- 
ing the  characters  AB,  Ab,  Ba,  and  db.  These  hybrids, 
on  crossing  haphazard,  would  yield  the  following: 

(AB  +  Ab  +  Ba  +  o&)2  =  A*B*  +  AW  +2A*Bb  +  2AB*a  + 

38  35  65  60 

4  ABdb  +2  Aab*  +  2  Ba?b  +  BW  +  «S69 
138  67  68         28        30 

The  figures  underneath  indicate  the  actual  numbers 
of  plants  obtained  by  Mendel  from  the  556  seeds 
yielded  by  the  15  original  hybrid  plants.  The  average 
numbers  with  two,  three,  and  four  characters  are  re- 
spectively 34,  65,  and  138,  or  very  nearly  in  the  theo- 
retical proportion  of  1 :2 :4. 

Mendel  even  took  the  immense  trouble  to  cross 
parents  differing  in  respect  of  three  characters,  and  he 
found  that  the  offspring  of  the  resulting  hybrids  with 


BLASTOGENIC  VARIATIONS.  159 

3,  4,  5,  and  6  characters  were  on  an  average  respectively 
as  10  :  19  :  43  :  78,  or  very  nearly  as  1:  2:  4:  8.  He 
also  confirmed  his  law  by  some  observations  on  PJiase- 
olus  vulgaris  and  P.  nanus,  but  the  crossings  of  P. 
nanus  9  with  P.  multiflorus  gave  only  a  partial  result, 
whilst  those  on  Hieracium  did  not  agree  at  all.  In  the 
light  of  the  hybridisation  experiments  of  Kolreuter, 
Gartner,  and  others,  Mendel  recognised  that  his  law  was 
by  no  means  universally  applicable.  It  obviously  can 
only  apply  to  cases  of  exclusive  inheritance,  and  not  to 
those  of  blended  or  mixed  inheritance. 

De  Yries  *  has  made  similar  observations  to  those  of 
Mendel  upon  varieties  of  no  less  than  15  different 
species  of  plants,  and  in  every  case  found  that  the  pro- 
portion of  recessive  forms  obtained  in  the  second  gen- 
eration was  approximately  the  theoretical  25  per  cent. 
When  the  observations  were  continued  through  other 
generations,  the  results  likewise  agreed  with  theory. 
Tschermakf  repeated  MendePs  observations  upon 
the  different  varieties  of  Pisum  sativum,  and  with 
some  of  them  obtained  a  similar  result.  However, 
he  found  that  in  some  other  cases  $  the  law  did 
not  hold.  Correns  §  also  experimented  with  varie- 
ties of  the  pea,  and  he  found  that  whilst  some  of 
the  characters  obeyed  Mendel's  law,  others,  such  as 
the  colour  of  the  skin  of  the  seed,  did  not.  He  ob- 
tained a  similar  result  If  on  crossing  Matthiola  incana 

*Ber.  d.  deutsch.  Botan.  Gesell.,  xviii.  p.  83,  1900.  Translation 
in  J.  Roy.  Horticult.  Soc.,  xxv.  p.  243,  1901. 

fBer.  d.  deutsch.  Bot.  Ges.,  xviii.  p.  232, 1900. 

JBer.  d.  deutsch.  Bot.  Ges.,  xix.  p.  35,  1901;  also  Zeitschr.  f. 
d.  landwr.  Versuchewesen  in  Oesterr.,  iii.  p.  465,  1900. 

§Ber.  d.  deutsch.  Bot.  Ges.,  xviii.  p.  158,  1900. 

IBot.  Centralb.,  Ixxxiv.  p.  97,  1900. 


160  BLASTOGENIC   VARIATIONS. 

and  M .  glabra.  Thus  some  of  the  characters,  such  as 
the  colour  of  the  flower  petals,  remained  fixed  in  the 
hybrids.  Also,  on  the  whole,  the  hybrids  had  a  greater 
resemblance  to  the  female  than  to  the  male  parent. 
In  the  second  generation  flowers  of  new  colours,  viz., 
white  and  red,  appeared,  in  addition  to  the  yellow-white 
and  violet  flowers  exhibited  by  the  parents.  Correns 
also  made  a  number  of  crosses  between  undoubted 
species  (e.  g.,  Cirsium  palustre  .+.  spinosissimum, 
Achillea  macrophylla  +  moschata,  Car  ex  echinata  +, 
fcetida,  etc.)  and  he  is  doubtful  whether  one  of  these 
hybrids  showed  a  single  really  dominant  character.  It 
was  quite  obvious  that  almost  all  the  characters  which 
served  to  differentiate  the  parents  were  present,  in 
greater  or  less  degree,  in  the  hybrids.  Correns  con- 
cludes, therefore,  that  almost  without  exception  the 
domination  of  a  character  shows  itself  only  in  crosses 
between  varieties,  whilst  the  hybrids  of  true  species 
show  the  characters  of  both  species,  though  in  dimin- 
ished degree. 

It  has  been  pointed  out  by  Weldon  *  that  Mendel's 
results  are  partly  vitiated  by  the  fact  that  he  quite  neg- 
lected the  ancestry  of  the  plants  with  which  he  started 
his  cross-fertilisations.  Weldon  also  adduces  a  con- 
siderable body  of  evidence  to  show  that  the  separation 
of  the  seed  characters  into  definite  dominant  and  re- 
cessive types  by  no  means  invariably  holds  good.  The 
offspring  of  cross-bred  peas  may  continue  to  contain  a 
large  percentage  of  intermediate  forms,  even  as  long  as 
25  generations  after  the  crossing. f 

*Biometrica,  i.  p.  228,  1902. 

*  For  further  evidence  concerning  the  Law  see  Report  to  Evolution 
Committee  of  Royal  Society  by  Miss  Saunders  and  W.  Bateson,  1902. 


BLASTOGENIC  VARIATIONS.  161 

According  to  Darwin,  variability  is  especially  induced 
if  mongrels  are  repeatedly  crossed  with  either  pure 
parent  form,  whilst  the  crossing  of  different  species 
may  lead  to  much  wider  variatio'n  than  the  crossing  of 
varieties.  The  hybrids  produced  on  the  first  cross  are, 
as  a  rule,  fairly  constant  in  their  characters,  but  if  these 
hybrids  be  crossed  again,  or  crossed  with  either  pure 
parent  form,  then  a  very  considerable  variability  may 
result.  "  He  who  wishes,"  says  Kolreuter,  "  to  obtain 
an  endless  number  of  varieties  from  hybrids,  should 
cross  and  recross  them."  *  Again  Darwin  t  says  that 
cross-bred  animals  "  for  breeding  are  found  utterly 
useless;  for  though  they  may  themselves  be  uniform  in 
character,  they  yield  during  many  generations  aston- 
ishingly diversified  offspring."  Indeed  it  would  seem 
that  entirely  new  characters  may  be  produced  by  this 
means.  For  instance,  Kolreuter  says  that  hybrids  in 
the  genus  Mirabilis  vary  almost  infinitely,  and  he  de- 
scribes new  and  singular  characters  in  the  seeds,  an- 
thers, and  cotyledons.  Professor  Lecoq  also  asserts 
that  many  of  the  hybrids  from  Mirabilis  jalapa  and 
multi-flora  might  easily  be  mistaken  for  distinct  species. 
Again,  Herbert  J  has  described  certain  hybrid  Khodo- 
dendrons  as  being  unlike  all  others  in  foliage,  just  as  if 
they  were  a  separate  species. 

According  to  Focke,§  the  hybrid  may  be  related  to 
the  parent  forms  in  three  different  ways:  (1)  there  may 

*  "  Animals  and  Plants,"  ii.  p.  254. 
\L.  c.,  ii.  p.  74. 

j  Science  Progress,  vol.  vii.  p.  185,  1898. 

§"Die  Pflanzen-Mischlingen,"  Berlin,  1881.  Quoted  by  Weis- 
mann,  "  Germ  Plasm,"  p.  261. 


162  BLASTOGENIC   VARIATIONS. 

be  a  strict  mean  in  all  parts;  (2)  the  paternal  or  mater- 
nal characters  may  predominate;  (3)  the  paternal  char- 
acters may  predominate  in  some  parts  of  the  hybrid, 
and  the  maternal  in  others.  The  first-mentioned  con- 
dition is  by  far  the  most  frequent.  For  instance,  K61- 
reuter  states  that  the  hybrid  between  Nicotiana  rustica 
9  and  N.  paniculata  $  (tAVO  species  of  tobacco  plant) 
is  exactly  intermediate  between  the  parent  forms.  On 
the  other  hand,  the  hybrid  between  N.  paniculata  9 
and  N.  vincoeflora  $  bears  so  close  a  resemblance  to 
the  second  of  these  species  that  the  characters  of  N. 
paniculata  can  hardly  be  recognised  at  all.  An  in- 
stance of  the  third  class  is  occasionally  found  in  the 
cross  between  N.  rustica  2  and  N.  paniculata  $  ,  the 
blossoms  resembling  one  parent  species,  and  the  leaves 
the  other.  Again,  Milardet  *  has  obtained  a  series  of 
non-separating  crosses  by  the  union  of  Fragaria,  Rubus, 
etc.  They  resembled  either  the  male  or  the  female 
parent.  De  Vries  f  has  obtained  a  similar  result  with 
(Enothera  muricata  9  X  biennis,  which  displayed  the 
paternal  character.  Crosses  between  (Enothera  La- 
mar  clciana  $  and  0.  nanella  $  gave  progeny  which 
always  displayed  two  types,  the  maternal  and  paternal, 
but  these  occurred  in  very  varying  ratios.  Crosses  of 
0.  lata  9  and  0.  Lamarckiana  $  also  yielded  progeny 
of  both  parental  types. 

Plant  hybrids  are  of  considerably  more  frequent  oc- 
currence in  nature  than  animal  hybrids,  and,  by  virtue 
of  the  fertility  which  they  often  possess  are  of  dis- 

*Mem.  Soc.  Sc.  Phys.  et.  Nat.  Bordeaux,  vol.  iv.  p.  1,  1894. 
fBer.  d.  deutscli.  Bot.  Ges.,  xviii.  p.  435,  1900.     Translation  in 
J.  Roy.  Hort.  Soc.,  xxv.  p.  249,  1901. 


BLASTOGENIC  VARIATIONS.  163 

tinct  importance  as  a  source  of  variations.  Thus  Ben- 
nett,* in  a  paper  on  Hybridity  in  Plants,  makes  the  fol- 
lowing remark:  "  There  seems,  however,  scarcely  to  be 
room  for  doubt  that  in  some  of  our  abundant  wild 
genera,  such  as  Rubus,  8alix,  and  Hieracium,  hybridity 
is  not  uncommon  in  nature.  It  has  long  been  known 
that  in  some  genera,  such  as  Passiflora,  and  in  some 
Orchidese,  the  ovules  appear  to  be  even  more  readily 
fertilised  by  pollen  of  a  different  species.  W.  Focke 
now  states  that  this  is  also  the  case  with  the  species  of 
Lilium  belonging  to  the  group  lulbiferum,  and  with 
some  species  of  Hemerocallis;  and  J.  H.  Wilson  affirms 
the  same  respecting  the  Cape  genus  Albuca,  also  belong- 
ing to  the  Liliacese."  Again,  Rolfe  f  states  that  Nar- 
cissus incomparabilis  is  known  to  be  wild  in  France,  and 
that  Herbert  found  that  on  crossing  a  daffodil  with  pol- 
len of  N.  poeticus,  the  seedlings  yielded  flowers  identi- 
cal with  those  of  N.  incomparabilis.  Similarly,  by 
crossing  the  Daffodil  with  the  Jonquil,  Herbert  suc- 
ceeded in  producing  N.  odorus.  Again,  Engleheart 
has  proved  the  hybrid  origin  of  N.  biflorus  by  crossing 
N.  poeticus  with  the  pollen  of  N.  tazetta,  he  obtain- 
ing seedlings  identical  with  wild  forms.  Also  he 
reconstructed  N.  pulcJiellus  Salisb.,  by  crossing 
N.  triandrus  with  the  Jonquil,  the  seedlings  proving 
absolutely  identical  with  the  wild  plant.  Further 
"  Digitalis  supplies  some  wild  hybrids  whose  origin  has 
been  artificially  demonstrated.  For  example,  D.  pur- 
purascensy  Both,  has  been  reconstructed  by  crossing 
and  recrossing  D.  lutea  and  D.  purpurea;  and  D.  media, 

*Nat.  Sci.,vol.  ii.  p.  208. 

\  J.  Roy.  Horticult.  Soc.,  xxiv.  p.  181,  1900. 


164  BLASTOGENIC  VARIATIONS. 

Roth,  in  the  same  way  from  D.  purpurea  and  D.  am- 
bigua  (grandiflora)  .  .  .  D.  ambigua  has  also  been 
crossed  with  D.  purpurea  and  with  D.  lanata,  in  each 
case  yielding  hybrids  which  also  occur  wild." 

Rolfe  also  records  that  Wichura  succeeded  in  raising 
artificially  no  less  than  eight  hybrid  willows  identical 
with  those  which  had  long  been  known  in  the  wild  state, 
and  Linton  has  added  at  least  six  others.  For  instance, 
Salix  rubra  was  obtained  by  crossing  8.  purpurea  with 
the  pollen  of  S.  viminalis. 

Kerner  is  of  the  opinion  that  species  may  be  produced 
by  hybridisation.  In  his  "  Natural  History  of  Plants  " 
he  gives  instances  of  these  hybrid  races.  To  quote 
Rolfe,  "  A  hybrid  between  Medicago  falcata  and  sativa, 
known  as  M.  media,  is  widely  cultivated  as  a  fodder 
plant,  and  is  propagated  from  seed.  Salvia  betonicce- 
folia,  a  hybrid  from  8.  nemorosa  and  nutans,  is  as  com- 
mon as  its  parents  in  grassland  in  Central  Hungary. 
Betula  alpestrisj  a  hybrid  between  B.  alba  and  nana,  is 
abundant  in  the  Jura,  Scandinavia,  and  in  North  Russia, 
here  and  there  whole  copses  of  it  being  found.  Nigri- 
tella  suaveolens,  a  hybrid  between  N.  angustifolia  and 
Gymnadenia  conopsea,  is  abundant  in  some  Swiss  locali- 
ties, hundreds  of  plants  sometimes  occurring  in  a  single 
meadow.  Hybrids  between  the  Primrose  and  Cowslip 
occur  in  thousands  in  upland  meadows  in  the  Eastern 
Alps." 

Again,  in  some  localities  in  the  Tyrol  the  hybrid 
Rhododendron  intermedium  exists  side  by  side  with  its 
parent  forms,  R.  ferrugineum  and  R.  hirsutum,  it 
sometimes  being  commoner  than  they  are.  Also  it 
seeds  freely,  and  comes  true  to  seed,  and  so  fulfils  all 


BLASTOGENIC  VARIATIONS.  165 

the  requirements  of  a  species.  The  same  is  true  of 
Salvia  sylvestris,  a  hybrid  from  8.  nemorosa  and  fira- 
tensis,  which  abounds  in  dry  meadows  all  over  the  low 
country  south  of  Vienna,  and  of  Nuphar  intermedium, 
a  hybrid  from  N.  luteum  and  pumilum,  which  occurs  in 
the  Black  Forest,  Russia,  Sweden,  and  other  localities. 
It  appears  that  a  hybrid  is  sometimes  found  in  company 
with  one  parent  only,  or  with  one  in  one  locality  and 
both  in  another;  or  sometimes  even  where  both  are 
absent. 

Kerner  estimated  that  something  like  a  thousand 
natural  hybrids  have  been  found  in  Europe  during  the 
last  forty  years,  but  of  these  hybrids  only  a  fraction 
survive  and  multiply. 

As  regards  artificial  hybrids,  Hurst  *  has  compiled  a 
list  of  genera  from  various  authorities,  and  from  his 
own  observations,  and  he  finds  that  91  distinct  genera 
are  recorded  in  which  fertile  hybrids  are  known.  In 
only  three,  viz.,  Ribes,  Polemonium,  and  Digitalis,  were 
the  hybrids  all  quite  infertile,  and  in  none  of  them  had 
many  experiments  been  made. 

Hurst  also  remarks  that  "  during  the  past  seven  years 
Mr.  Reginald  Young  has  been  crossing  inter  se  some  30 
distinct  species  and  53  distinct  hybrids  in  the  genus 
Paphiopedilum  (Pfitz),and  has  .  .  .  carefully  recorded 
no  less  than  849  crosses.  Of  these,  taken  together, 
80.2  per  cent,  have  proved  fertile,  i.  e.,  produced  good 
seeds.  Of  263  crosses  between  distinct  species,  95  per 
cent,  were  fertile.  This  seems  to  show  that  in  this 
genus  crosses  between  distinct  species  are  almost,  if  not 
quite,  as  fertile  as  crosses  between  varieties  of  the  same 
*  J.  Roy.  Horticult.  Soc.,  xxiv.  p.  90,  1900. 


166  BLASTOQENIC   VARIATIONS. 

species;  while  in  crosses  in  which  a  hybrid  was  con- 
cerned in  the  parentage,  out  of  586,  only  73.5  per  cent, 
proved  fertile,  showing  that  crosses  with  hybrids, 
though  fertile  to  a  high  degree,  are  yet  rather  less  fer- 
tile than  crosses  between  species.  .  .  A  further  analysis 
of  the  figures  shows  that  while  hybrids  crossed  with  the 
pollen  of  pure  species  give  91.8  per  cent,  fertile,  yet 
pure  species  crossed  with  the  pollen  of  hybrids  give  but 
60  per  cent,  fertile."  That  is  to  say,  the  decline  in  the 
fertility  of  the  hybrids  is  due  in  a  large  measure  to  the 
loss  of  power  in  the  pollen  of  the  hybrids.  This  decline 
in  power  of  the  male  element  has  been  noticed  before 
in  other  plants  by  Darwin,  Focke,  and  others. 

Rimpau  *  has  made  series  of  experiments  on  the 
crossing  of  some  of  our  common  agricultural  plants,  and, 
amongst  other  results,  obtained  ten  artificial  and  nine 
natural  hybrids  in  wheat,  and  two  artificial  and  six 
natural  hybrids  in  barley.  His  most  striking  result  of 
all  was  to  obtain  a  fertile  hybrid  between  wheat  and 
rye,  plants  belonging  to  different  genera.  Again, 
Hurst  states  f  that  amongst  Orchids  no  less  than  150 
bigeneric  crosses  are  recorded.  Bigeneric  hybrids  have 
also  been  recorded  J  between  Philesia  and  Lapigeria, 
between  Urceolina  and  Eucharis,  between  numerous 
genera  of  Gresneracese,  etc.  Finally  a  cross  has  been 
described  §  from  Digitalis  ambigua  (Scrophulariacese) 
by  pollen  of  Sinningia  speciosa  (Gesneracese) ;  i.  e.,  a 
binordinal  hybrid. 

*  ' '  Kreutzungsproducte  landwirth.   Cultur-pflanzen,"  Berlin ,  1891. 

\Loc.  cit. 

i  Nature,  vol.  Ixiv.  p.  447,  1901. 

§Maund's  "  Botanic  Garden,"  v.  p.  468. 


BLASTOGENIC  VARIATIONS.  167 

Upon  members  of  the  Animal  Kingdom  very  few  ex- 
tensive and  systematic  crossing  experiments  have  been 
made.  The  most  complete  are  those  of  Standfuss,*  on 
various  races  and  species  among  the  Lepidoptera. 
Standfuss'  general  conclusion  is  that  on  crossing  the 
normal  form  of  a  species  with  a  gradually  formed  local 
race  of  the  same  species,  a  series  of  more  or  less  inter- 
mediate forms  results.  For  example,  on  crossing  Calli- 
morpha  dominula  $  with  the  variety  persona  9,  the 
issue  resulting  were  of  a  very  variable  form,  more  or 
less  intermediate,  but  somewhat  more  closely  resem- 
bling the  type  than  the  variety.  In  the  reciprocal 
cross,  the  insects,  on  the  whole,  also  came  nearer  to  (7. 
dominula  than  to  the  variety,  but  not  so  much  as  be- 
fore. When  species  were  crossed  Standf uss  found  that 
the  hybrid  form  lay  between  the  extreme  parental 
forms,  but  was  not  strictly  intermediate.  Arguing 
from  his  experiments  on  crossing  various  species  of 
Saturnia,  Standfuss  concludes  that  the  adult  offspring 
are  more  similar  to  the  male  parent  than  to  the  female, 
the  extent  of  approximation  depending  on  the  relative 
age  of  the  two  species.  Crosses  of  the  male  hybrids 
with  the  parent  forms  were  in  some  cases  proved  to  be 
fertile,  and  hence  there  is  no  reason  why  such  forms 
should  not  establish  themselves  under  natural  condi- 
tions. Thus  Dr.  Dixey,  in  a  very  good  resume  of 
Standfuss'  researches,f  says  with  reference  to  these  hy- 
brids, "  Since  the  product  of  this  kind  of  crossing  is  not 
found  to  show  a  complete  reversion  to  the  type  of  the 
female  parent,  it  is  possible  that  the  existence  of  vari- 

*"  Handbuch  der  palaartischen  Gross-Schmetterlinge,"  Jena,  1896. 
t  Science  Progr.,  vol.  vii.  p.  185,  1898. 


168  BLASTOGENIC   VARIATIONS. 

ous  intermediate  forms  in  such  genera  as  Melitcea, 
Zygoma,  and  Agrotis  may  be  accounted  for  in  this  man- 
ner. Cases  of  simple  pairing  between  distinct  species 
of  the  two  former  genera  have  been  observed  by  the 
author  [Standfuss]  in  nature." 

Upon  Echinoids,  the  author  has  made  numerous 
crosses  and  reciprocal  crosses.*  Eight  species  were 
worked  with,  and  of  the  56  possible  crosses,  41  were  at- 
tempted. Of  these,  22  yielded  larvae  of  8  days'  growth. 
In  only  one  cross  did  any  of  the  larvae  incline  towards 
the  paternal  type,  and  the  majority  of  those  then  ob- 
tained were  more  or  less  intermediate.  In  nine  other 
crosses  also  they  were  more  or  less  intermediate  in  char- 
acter, whilst  in  the  remaining  twelve  they  were  of  the 
maternal  type.  A  few  of  the  larvae  exhibited  char- 
acters which  were  not  present  in  either  parent. 

Upon  members  of  the  Mammalian  and  Avian  King- 
doms, a  very  large  number  of  crossing  experiments 
have  been  made,  and  frequently  with  success,  but  the 
observations  are  not  sufficiently  extensive  to  admit  of 
generalisations.  The  most  interesting  experiments  of 
recent  years  are  those  of  Professor  Ewart,  upon  zebra 
hybrids.f  By  crossing  mares  of  various  sizes  (11  to  15 
hands)  with  a  zebra  stallion,  nine  hybrids  were  obtained 
altogether.  Also  Professor  Ewart  had  in  his  possession 
three  hybrids  out  of  zebra  mares,  one  having  for  his 
sire  a  donkey,  whilst  the  other  two  were  sired  by  ponies. 
The  hybrids  showed  a  "  curious  blending  of  characters, 
derived  apparently  partly  from  their  actual  and  partly 

*Phil.  Trans.  Roy.  Soc.,  1898,  B.  p.  483,  and  Arch,  f .  Entwick- 
elungsmechanik. ,  Bd.  ix.  p.  468,  1900. 
f  "  The  Penycuik  Experiments." 


BLASTOGENIC  VARIATIONS.  169 

from  their  remote  ancestors.  .  .  Some  of  the  hybrids 
in  make  and  disposition  strongly  suggest  their  zebra 
sire,  others  their  respective  dams;  but  even  the  most 
zebra-like  in  form  are  utterly  unlike  their  sire  in  their 
markings."  In  some  respects,  also,  the  hybrids  were 
intermediate  between  their  parents. 

As  to  the  causes  of  the  different  relationships  be- 
tween parental  and  hybrid  characters,  we  are  almost 
entirely  in  the  dark.  Weismann  has  endeavoured  to 
account  for  them  on  his  theory  of  the  germ-plasm,  but 
his  explanation  is  purely  theoretical  and  from  its  nature 
incapable  of  experimental  verification.  The  observa- 
tions of  the  author  on  sea-urchin  hybrids,  and  of  Pro- 
fessor Ewart  on  crosses  between  varieties  of  rabbits, 
throw  a  little  light  on  the  subject,  for  they  show  that 
the  characters  of  the  hybrids  may  be  considerably  in- 
fluenced by  the  seasonal  condition  of  the  parental  sex- 
cells,  and  thereby  seem  to  indicate  that  the  compara- 
tive degrees  of  nutrition  of  the  sex-cells,  and  perhaps 
also  of  their  constituent  parts,  may  be  a  very  important 
factor.  One  should  also  bear  in  mind  that,  as  was 
demonstrated  by  Mendel  in  the  case  of  certain  plant 
hybrids,  some  of  the  parental  characters  may  remain 
latent  in  the  hybrid  offspring,  and  only  reveal  their 
presence  in  subsequent  generations.  The  existence  of 
latency  is  also  shown  by  secondary  sexual  characters. 
In  every  female  all  the  secondary  male  characters,  and 
in  every  male  all  the  secondary  female  characters,  ap- 
parently exist  in  a  latent  state,  ready  to  be  evolved 
under  certain  conditions,  such  as  the  removal  of  the 
ovaries  or  testes.  The  variability  of  hybrids  may  there- 
fore be  due  not  only  to  their  having  received  varying 


170  BLASTOGENIC   VARIATIONS. 

and  unequal  amounts  of  the  different  characters  from 
their  parents,  to  these  either  partially  or  entirely  refus- 
ing to  blend,  but  also  to  some  of  the  characters  received 
remaining  latent,  or  to  characters  latent  in  the  parents 
revealing  themselves  in  the  offspring. 

Sports.  Instances  of  so-called  sports,  or  suddenly 
occurring  aberrant  variations,  have  been  given  in  the 
second  chapter,  but  nothing  was  said  of  their  origin.  To 
what  are  we  to  attribute  this  ?  Are  they  to  be  regarded 
as  normal,  only  somewhat  exaggerated,  variations,  or 
are  they  something  essentially  different?  The  more 
general  opinion  probably  inclines  to  the  latter  view,  as 
there  are  several  facts  which  it  is  difficult  to  reconcile 
with  the  former.  It  is  said,  for  instance,  that  sports, 
as  distinguished  from  varieties,  are  much  more  stable; 
that  they  may  be  transmitted  to  successive  generations 
with  considerable  persistence  and  in  undiminished 
strength.  Galton  has  suggested  *  that  whilst  organ- 
isms showing  ordinary  variations  are  grouped  round  one 
"  position  of  organic  stability,"  towards  which  the  off- 
spring in  the  next  generation  tend  to  regress,  sports  are 
centred  round  a  different  position  of  stability,  and  are 
not  merely  a  strained  modification  of  the  original  type. 
They  therefore  have  little  tendency  to  revert  to  Ais 
original  type,  but  are  capable  of  propagating  their 
freshly  acquired  characters  more  or  less  undiminished, 
and  so  giving  rise  to  fresh  races.  Galton  considers  that 
the  results  which  he  has  obtained  in  his  detailed  study 
of  human  finger-prints  f  afford  strong  evidence  in  sup- 
port of  his  view.  These  patterns,  formed  by  the  papil- 

*  Vide  "Natural  Inheritance,"  p.  30;  also  "Mind,"  p.  362,  1894. 
fPhil.  Trans.  1891,  B. 


BLASTOQENIC  VARIATIONS.  171 

lary  ridges  on  the  bulbs  of  the  fingers,  are  the  most  per- 
sistent of  all  the  external  characters  that  have  yet  been 
examined.  They  are  found  to  fall  in  three  definite  and 
widely  different  classes.  Each  of  these  is  a  true  race 
in  the  sense  in  which  that  word  was  defined,  transitional 
forms  being  rare  and  the  typical  forms  being  frequent. 
Galton  thinks  that  the  continual  appearance  of  these 
well-marked  and  very  distinct  patterns  proves  the 
reality  of  the  alleged  positions  of  organic  stability. 

A  clear  distinction  between  sports  and  varieties  seems 
to  show  itself  also  amongst  the  Lepidoptera.  Thus 
Standfuss  *  found  that  when  a  sport  is  crossed  with  its 
parent  form,  the  issue  is  sharply  divided  in  both  sexes 
into  specimens  resembling  either  the  sport  or  the  nor- 
mal form.  There  are  no  true  intermediate  forms, 
though  occasionally  forms  are  observed  in  which  the 
characters  are  unsymmetrically  mixed.  When  the  nor- 
mal form  of  a  species  is  crossed  with  a  gradually  formed 
local  race,  however,  a  series  of  intermediate  forms  is 
obtained.  We  have  seen  also  that  De  Yries,  in  his  ex- 
periments on  plants,  claims  to  have  found  a  wide  dif- 
ference between  mere  varieties,  and  true  sports  such  as 
were  obtained  from  (Enothera  Lamarckiana. 

As  already  mentioned,  sports  have  been  stated  to  be 
much  more  persistent  in  propagating  their  aberrant 
characters  than  normal  varieties,  but  the  evidence  in 
favour  of  such  a  generalised  statement  is  quite  in- 
sufficient. There  are  certainly  a  few  instances  which 
strongly  support  it,  but  there  are  a  good  many  more 
which  entirely  fail  to  do  so.  Of  the  former,  the  in- 

*"Handbuch   der  palaartischen    Gross-Schmetterlinge,"    Jena, 
1896. 


172  BLASTOGENIC   VARIATIONS. 

stances  of  the  ancon  or  otter  sheep  *  and  japanned  or 
black-shouldered  peacocks,f  quoted  by  Darwin,  are  the 
most  striking.  The  originator  of  the  ancon  breed  of 
sheep  was  a  single  ram,  born  in  Massachusetts  in  1791. 
Ancon  rams  and  ewes  invariably  produced  ancon  off- 
spring, whilst  when  crossed  with  other  breeds  the  off- 
spring resembled  either  parent,  and  only  very  excep- 
tionally yielded  intermediate  forms.  Japanned  pea- 
cocks, which  differ  conspicuously  from  the  common 
peacock  in  colouring,  appear  suddenly  in  flocks  of  the 
common  kind.  Though  smaller  and  weaker  birds,  they 
have  been  known  in  two  instances  to  increase,  and  finally 
extinguish  the  previously  existing  breed.  They  would 
therefore  seem  to  have  been  strongly  prepotent. 

That  sports  may  be  no  more  transmissible  than  other 
variations  seems  to  be  true  in  the  case  of  polydac- 
tylism,  for  Dr.  Struthers  asserts  that  cases  of  non- 
inheritance  and  of  the  first  appearance  of  additional 
digits  in  unaffected  families  are  much  more  frequent 
than  cases  of  inheritance. {  Again,  Galton  regards  as 
sports  the  mental  arithmeticians  and  eminent  musi- 
cians who  are  occasionally  born  into  families  which  in 
previous  generations  have  shown  no  signs  of  such  ex- 
ceptional characters.  Though  these  characters  may 
be  transmitted  to  descendants,  yet  this  is  the  excep- 
tion, and  not  the  rule.  The  subservience  of  sports 
to  the  law  of  hereditary  transmission  is  well  shown 
by  some  observations  of  Standfuss  on  Lepidop- 
tera.  In  1888  a  normal  female  Aglia  tau  was  crossed 

*  "  Animals  and  Plants,"  i.  p.  104. 
\L.  c.,  i.  p.  305. 
JZ.  c.,i.  p.  458. 


BLASTOGENIC  VARIATIONS.  173 

with  a  dark  aberrant  form  or  sport  of  this  species, 
Aglia  lugens,  which  had  been  interbred  for  two  genera- 
tions. In  1889  some  of  the  lugens,  both  male  and 
female,  obtained  from  this  cross,  were  crossed  with 
normal  tau  specimens.  About  half  the  offspring  ob- 
tained resembled  one  parent  and  half  the  other,  inter- 
mediate forms  being  absent.  On  breeding  some  of  the 
1889  9  and  $  lug  ens  together,  however,  their  off- 
spring consisted  of  about  36  per  cent,  of  tau,  and  64  per 
cent,  of  lugens  forms.  In  1890  some  of  these  lugens 
were  bred  together,  and  their  offspring  consisted  almost 
entirely  of  lugens,  only  11  per  cent,  being  of  the  tau 
form.  In  this  latter  case,  therefore,  both  parents  and 
all  four  grandparents  were  lugens;  in  the  1889  off- 
spring, both  parents  but  only  two  grandparents,  and  in 
the  1888  offspring  only  one  parent  and  two  grand- 
parents. 

If  sports  be  of  an  essentially  different  nature  to  nor- 
mal variations,  as  the  somewhat  insufficient  evidence 
available  may  perhaps  be  taken  to  indicate,  how  is  it 
that  they  arise  ?  Apparently  they  occur  spontaneously, 
but  doubtless  some  exciting  cause  must  exist,  invisible 
though  it  may  be.  The  artificial  production  of  mon- 
sters seems  to  throw  some  light  on  the  subject,  and 
hence  a  brief  reference  to  them  may  be  made.  These 
monsters  or  malformations  probably  differ  from  sports 
only  in  degree,  and  not  in  kind.  Hence,  if  the  means 
adopted  for  their  artificial  production  are  such  as  may 
occur  under  natural  conditions,  it  seems  possible,  and 
even  probable,  that  sports  themselves  may  owe  their 
origin  to  similar  agencies.  For  instance,  Dareste,  as 
long  ago  as  1877,  described  numerous  experiments  on 


174  BLASTOGENIC   VARIATIONS. 

the  effects  of  placing  fowls'  eggs  vertically  instead  of 
horizontally  during  development,  of  keeping  them 
slightly  above  or  below  the  normal  temperature  of  incu- 
bation, of  heating  different  parts  of  the  egg  unequally, 
and  of  modifying  the  conditions  of  respiration  by  var- 
nishing part  of  the  shell.*  Various  considerable  mal- 
formations were  produced,  but  these  were  more  or  less 
the  same,  whatever  the  conditions  that  produced  them. 
Professor  Windle,t  who  has  extended  these  investiga- 
tions and  determined  the  effects  of  various  other 
agencies,  as  electricity  and  magnetism^  on  development, 
came  to  a  similar  conclusion.  He  considered  that  these 
disturbing  agents  act,  in  the  majority  of  cases,  on  that 
part  of  the  developing  organisation  which  is  concerned 
with  the  formation  of  the  vascular  system  of  the  em- 
bryo, and  so  indirectly  produce  the  malformations  ob- 
served. 

The  suggested  connection  between  considerable  mal- 
formations and  sports  has  not  as  yet  been  borne  out  by 
Dareste's  researches,  although  observations  have  been 
made  with  the  object  of  finding  it.  In  these  observa- 
tions the  conditions  found  to  produce  considerable  mal- 
formations were  reduced  in  strength,  in  the  hope  of 
thereby  obtaining  only  slight  anomalies,  compatible 
with  continued  existence  and  the  procreation  of  off- 
spring. Unfortunately  the  domestic  fowl,  which  was 
invariably  made  use  of,  is  unsuitable  for  such  observa- 
tions. The  type  is  so  diversified  that  the  experimenter 
who  obtains  some  variety  can  never  be  certain  whether 

* "  Recherches  sur  la  Production  artificielle  des  Monstrosites," 
Paris,  1877;  second  edition,  1891. 
fProc.  Birmingham  Phil.  Soc.,  vii.  p.  220,  1890. 


BLASTOGENIC  VARIATIONS.  175 

it  should  be  attributed  to  the  conditions  of  experiment, 
or  to  some  physiological  cause  arising  in  the  egg  itself. 
To  test  this  question  with  some  chance  of  success,  the 
eggs  of  some  species  which  varies  but  little  ought  to 
be  employed;  e.  g.,  some  wild  species.  But  in  this  case 
it  would  be  very  difficult  to  obtain  sufficient  material. 

In  the  case  of  certain  Lepidoptera,  however,  the  arti- 
ficial production  of  sports  has  been  successfully  accom- 
plished by  Standfuss.*  By  keeping  the  pupae  of  V. 
cardui  (Painted  Lady)  at  a  high  temperature  for  a 
short  period,  he  succeeded  in  producing  a  small  number 
of  specimens  of  the  aberrant  form  elymi,  a  form  which 
is  occasionally  found  under  natural  conditions.  Again 
a  low  temperature,  acting  on  pupae  of  V.  io  (Peacock), 
produced  a  variety  ab.  fischeri,  which  exhibits  a  reduc- 
tion in  the  number  of  the  blue  scales  on  both  fore  and 
hind  wings.  In  these  and  other  characters  there 
seemed  to  be  an  approach  to  the  type  of  V.  urticce. 
These  and  other  observations  seem  to  justify  Standfuss' 
conclusion  that  many  of  the  aberrations  occurring  in 
nature  may  likewise  have  arisen  through  the  influence 
of  abnormal  temperature  conditions. 

Telegony.  The  term  telegony,  or  so  called  infection 
of  the  germ,  is  applied  to  certain  cases  apparently  show- 
ing the  influence  of  a  previous  fertilisation  on  the 
structure  of  the  subsequent  offspring.  The  test  case, 
always  quoted  in  support  of  the  existence  of  this  phe- 
nomenon, is  that  of  Lord  Morton's  mare.  This  animal 
bore  a  hybrid  to  a  quagga,  and  subsequently  produced 
two  colts  by  a  Black  Arabian  horse.  These  colts,  both 
in  the  hair  of  their  manes,  their  partial  dun  colour,  and 
*The  Entomologist,  vol.  xxviii.  p.  145,  1895. 


176  BLASTOGENIC   VARIATIONS. 

striping  on  the  legs,  strongly  resembled  the  quagga. 
Other  cases  have  been  quoted  in  support  of  this  phe- 
nomenon, but  it  is  unnecessary  to  mention  them  here, 
for  none  of  them  are  absolutely  convincing.  The 
reader  who  wishes  for  details  of  these  cases  should  con- 
sult a  useful  paper  by  Finn.*  In  his  Penycuik  experi- 
ments, Professor  Ewart  has  made  a  number  of  attempts 
to  obtain  evidence  of  the  phenomenon,  but  so  far  with 
entirely  negative  results.  Sir  Everett  Millais  made  a 
considerably  larger  series  of  experiments,  on  a  variety 
of  animals,  but  was  equally  unsuccessful.  Many  Ger- 
man breeders  also  believe  telegony  as  yet  unproven. 
Finally,  Professor  Pearson  f  has  shown  that  exact 
statistical  examination  of  appropriate  data  gives  no 
support  whatever  to  the  hypothesis.  Pearson's  method 
of  testing  the  question  was  to  determine  whether 
younger  children  are  more  closely  correlated  to 
their  parents  in  respect  of  some  character  such  as 
stature,  than  older  children.  Supposing  the  male 
parent  were  able  to  exert  any  influence  on  the  ma- 
ternal tissues,  and  so  indirectly  on  the  offspring, 
then  clearly  this  influence  would  be  greater  for  the 
younger  children  than  for  the  older  children.  As 
Pearson  recognises,  it  is  possible  that  telegony,  if  it 
occurs  at  all,  is  due  to  the  abnormal  preservation  of  the 
male  sex  cells  of  an  earlier  union,  and  in  such  a  case  his 
method  would  afford  no  evidence  one  way  or  the  other. 
Probably,  therefore,  no  such  thing  as  telegony  exists. 
In  any  case  it  is  so  exceedingly  rare  that,  as  a  possible 
source  of  variations,  it  may  be  neglected. 

*Nat.  Sci.,  vol.  iii.  p.  436. 
fProc.  Roy.  Soc.,  Ix.  p.  273. 


BLASTOGENIC  VAEIATIONS.  177 

Parthenogenesis.  We  saw  in  the  last  chapter  that 
Weismann  regarded  sexual  reproduction  as  a  potent 
factor  in  the  production  of  variations,  in  that  it  af- 
forded inexhaustible  supplies  of  fresh  combinations  of 
the  individual  variations  already  represented  in  the 
mingling  germ-plasms.  We  should  accordingly  con- 
clude that  when  such  sexual  union  is  wanting,  as  in 
parthenogenetically  produced  animals,  the  amount  of 
variation  will  be  smaller,  and  that  parent  and  offspring 
will  more  closely  resemble  each  other.  The  evidence 
upon  this  point  is  exceedingly  slight,  but  what  there  is 
perhaps  tends  rather  to  support  this  deduction.  Thus 
Weismann  made  a  series  of  observations,  extending 
over  eight  years,  upon  a  small  ostracod,  Cypris  reptans. 
This  organism  exists  as  two  well-marked  varieties,  one 
being  coloured  yellow,  with  five  small  green  spots  on 
each  side  of  the  shell,  and  the  other  seemingly  dark 
green,  owing  to  the  great  enlargement  of  these  spots.* 
Both  varieties  are  produced  parthenogenetically  in  the 
neighbourhood  of  Freiburg,  males  never  being  found. 
Females  of  each  variety  were  isolated,  fed  well,  and 
allowed  to  multiply  for  many  generations.  It  was 
found  that  "  the  descendants  of  the  same  mother  re- 
sembled one  another  as  well  as  the  parent  with  which 
the  experiment  began,  even  as  regards  minute  details 
of  the  markings.  The  differences  were  mostly  as  small 
as  those  which  may  be  observed  in  identical  human 
twins."  Even  after  many  generations  no  modification 
showed  itself,  so  that  colonies  were  obtained  which 
could  not  be  distinguished  from  their  ancestors  40  gen- 
erations back.  In  three  different  instances,  however, 
*  '•  Germ-Plasm,"  p.  344. 


178  BLASTOGENIC  VARIATIONS. 

some  of  the  dark  green  variety  appeared  in  broods  of 
the  typical  yellow  variety,  and  in  one  instance  some  of 
the  yellow  variety  in  broods  of  the  dark  green.  These 
sudden  transformations  could  not  have  been  due  to  ex- 
ternal circumstances,  as  the  two  forms  appeared  in  the 
same  aquaria,  under  precisely  the  same  conditions. 
Weismann  attributes  them  to  reversion. 

Evidence  telling  in  the  opposite  direction  to  Weis- 
mann's  has  recently  been  obtained  by  Warren,*  and  as 
it  is  based  on  exact  statistical  measurements,  one  is  in- 
clined at  first  sight  to  attach  greater  weight  to  it.  The 
observations  consisted  in  measurements  of  the  total 
length  of  body  to  base  of  spine,  and  of  the  length  of  the 
protopodite  of  the  second  antenna  of  the  right  side,  in 
23  female  Daphnia  magna,  and  their  96  partheno- 
genetically  produced  offspring.  As  these  animals  con- 
tinue to  grow  throughout  life,  the  second  dimension 
was  expressed  in  terms  of  the  first,  before  calculating 
its  variability.  Its  error  of  mean  square,  or  standard 
deviation,  was  found  to  be  2.22  in  the  mothers,  and 
2.95  in  the  offspring.  That  is  to  say,  the  offspring  were 
distinctly  more  variable  than  the  mothers,  and  even  the 
offspring  of  a  single  mother  were  found  to  be  on  an 
average  more  variable  than  all  the  mothers  put  to- 
gether. As  the  mothers  had  in  a  way  been  selected, 
only  those  which  produced  offspring  being  chosen,  the 
daughters  would  be  expected  to  be  somewhat  more  vari- 
able, but  in  any  case  the  variability  was  considerable. 
Again,  it  was  found  that  the  coefficient  of  correlation 
between  mother  and  offspring  was  .446,  whilst  the  co- 
efficient of  regression  of  offspring  on  mothers  was  .619. 
*Proc.  Roy.  Soc.,  Ixv.  p.  154,  1899. 


BLASTOGENIC  VARIATIONS.  179 

Now  it  has  already  been  found,  in  the  case  of  stature  in 
man,  that  the  correlation  between  mid-parent  (i.  e., 
mean  between  male  and  transmuted  female)  and  off- 
spring is  .424,  and  the  regression  of  offspring  on  mid- 
parents  .6;*  hence  Warren's  values  seem  to  show  that 
the  parthenogenetic  mother  acts  as  a  mid-parent  to  her 
offspring,  and  not  as  a  single  parent,  and  also  that  these 
offspring  exhibit  regression  towards  the  mean  race  type, 
just  as  sexually  produced  individuals  do.  As  Warren 
himself  points  out,  however,  his  evidence  is  not  con- 
clusive. Thus  the  number  of  individuals  measured 
was  comparatively  small,  and  also  it  would  seem  that 
Daphnia  is  a  very  unreliable  organism  to  work  with. 
It  is  so  exceedingly  sensitive  to  its  environmental  con- 
ditions f — very  considerable  variations  being  produced 
by  comparatively  slight  changes — that  these  data  de- 
rived from  it  can  only  be  accepted  with  considerable 
reserve. 

A  further  series  of  observations  was  made  by  War- 
ren J  upon  Aphides  (Hyalopterus  irirhodus).  Sixty 
parents  and  their  368  children  were  measured,  and  also 
30  grandparents  and  their  291  grandchildren.  War- 
ren found  that  the  coefficients  of  parental  and  grand- 

*  It  has  been  stated  in  the  previous  chapter  that  the  coefficients  of 
correlation  and  of  regression  between  single  parent  and  offspring  are 
practically  the  same  thing,  and  are  equal  to  .3.  The  coefficient 
of  correlation  between  mid-parent  and  offspring  is,  however, 
/y/2  x  .3  =  .424,  because  the  mid-parent,  being  the  mean  of  two 
parents,  is  less  variable  than  the  single  parents  (in  the  proportion  of 

1  to  -/o).    The  coefficient  of  regression  of  offspring  on  mid-parents, 

is,  however,  twice  that  of  offspring  on  a  single  parent,  i.  e.,  is  .6. 
\Vide  Q.  J.  Microsc.  Sci.,  vol.  xliii.  p.  199,  1900. 
t  Biometrika,  i.  p.  129,  1902. 


180  BLASTOGENIC  VARIATIONS. 

parental  correlation  showed  no  marked  difference  from 
those  obtained  in  sexual  reproduction,  just  as  in  the 
case  of  his  daphnia  observations.  If  anything,  there 
was  a  decrease  in  the  correlation,  on  passing  from 
sexual  to  parthenogenetic  forms,  rather  than  the  in- 
crease we  should  expect.  However,  the  variability  of 
the  individuals  of  a  brood  was  found  to  be  only  about 
60  per  cent,  of  the  racial  variability;  -i.e.,  distinctly  less 
than  in  sexual  reproduction.  Also  the  mean  coefficient 
of  fraternal  correlation  for  aphis  and  daphnia  was  .66, 
or  considerably  higher  than  the  mean  value  of  .45  ob- 
tained by  Pearson  *  for  fraternal  correlation  among 
sexual  forms.  Warren's  general  conclusion  may  be 
summed  up  in  the  words:  "  The  question  as  to  whether 
we  have  a  real  difference  between  parthenogenetic  and 
sexual  offspring  can  only  be  decided  by  further  investi- 
gation both  on  aphis  and  other  forms."  In  the  light  of 
Weismann's  observations,  which  were  carried  on  for 
such  a  number  of  generations,  we  seem  entitled  to  con- 
clude that  probably  a  real  difference  will  be  found  to 
exist  between  them. 

Arguing  partly  from  Warren's  observations,  and 
partly  from  others  of  his  own,  Professor  Pearson  f  has 
come  to  the  conclusion  that,  "  whatever  be  the  function 
of  sex  in  evolution,  it  is  not  the  production  of  greater 
variability."  Thus  he  says  that  the  individual  contains 
in  itself  a  variability  which  is  80  to  90  per  cent,  of  the 
variability  of  the  race,  and  which  it  can  exhibit  quite 
independently  of  sexual  union,  e.  g.,  as  in  this  case  of 
parthenogenesis.  As  instances  of  individual  varia- 

*  Phil.  Trans.  1901.  A.  p.  285. 
f  "  Grammar  of  Science,"  p.  474. 


BLASTOGENIC  VARIATIONS.  181 

bility,  he  refers  to  the  stigmatic  bands  on  the  seed  cap- 
sules of  Shirley  poppies.  These  vary  in  number  from 
about  7  to  18,  the  most  commonly  occurring  number 
being  12.  The  variability  of  a  large  number  of  indi- 
viduals (as  expressed  by  the  error  of  mean  square), 
which  were  taken  as  a  good  sample  of  the  whole  race, 
was  found  to  be  1.885.  The  average  variability  of  the 
bands  in  the  capsules  obtained  from  each  of  300  differ- 
ent plants,  or  the  individual  variability,  was,  however, 
only  15  per  cent.  less.  Again,  with  reference  to  the 
number  of  leaflets  on  the  compound  leaf  of  the  ash,  the 
individual  variability  was  only  8  per  cent,  less  than  the 
racial  variability. 

In  a  recent  paper,*  Professor  Pearson  and  his  co- 
workers  have  determined  the  relationship  between 
racial  and  individual  variability  in  a  number  of  other 
plant  species.  Enumerations  were  made  of  the  veins 
in  the  leaf  of  the  Spanish  Chestnut  and  the  Beech,  of 
the  prickles  on  Holly  leaves,  the  sori  on  the  fronds  of 
Hartstongue  ferns,  the  seeds  in  the  pods  of  Broom 
plants,  etc.,  and  measurements  of  the  length  and 
breadth  of  ivy  leaves  and  of  the  gills  of  mushrooms. 
On  an  average,  the  individual  variability  was  found  to 
be  about  87  per  cent,  of  the  racial,  it  varying  in  the  dif- 
ferent series  of  observations  between  77  and  98  per 
cent.  Now,  even  admitting  that  in  these  instances  the 
individual  variability  is  only  slightly  smaller  than  the 
racial,  it  does  not  appear  to  me  that  Professor  Pearson  is 
entitled  to  his  contention,  for  all  these  highly  variable 
individuals  were,  of  course,  produced  as  the  result  of 
sexual  union  in  their  immediate  or  remote  progenitors. 
*  Phil.  Trans,  1901,  A.  p.  285. 


182  BLASTOGENIC   VARIATIONS. 

Such  union  may  have  been  the  starting  point  of  con- 
siderably increased  variation,  which  was  never  lost, 
even  through  innumerable  subsequent  asexual  genera- 
tions. Thus  Professor  Pearson  has  shown  that  if  the 
ancestors  of  individuals  be  selected  so  as  to  be  abso- 
lutely similar  in  character  for  an  indefinite  number  of 
generations  back,  such  individuals  will  still  have  a 
variability  of  upwards  of  89  per  cent,  of  that  of  the 
original  race.  Though  produced  sexually,  these  indi- 
viduals are  in  reality  comparable  to  asexually  repro- 
duced forms,  as  by  hypothesis  no  new  characters  were 
introduced  by  any  of  their  ancestors. 

"Whether  the  difference  between  racial  and  individual 
variability  is  as  small  as  Pearson  maintains,  or  not,  de- 
pends solely  on  what  is  meant  by  the  word  "  race."  If 
"  species,"  in  the  generally  accepted  sense,  is  meant, 
then  the  view  is  certainly  incorrect.  If,  however,  a  group 
of  individuals  is  implied,  all  of  which  have  been  exposed 
during  several  generations  to  practically  identical  con- 
ditions of  environment,  then  the  view  must  be  admitted. 
It  is  of  little  practical  value,  however,  as  may  be  real- 
ised by  quoting  certain  data  which  Pearson  has  himself 
brought  forward  in  another  connection.*  Thus  the 
variability  in  the  number  of  stigmatic  bands  in  a  sample 
of  the  wild  poppy,  Papaver  Ehceas,  collected  in  a  corn 
field  at  the  foot  of  the  Chiltern  Hills,  was  found  to  be 
1.473,  that  in  two  individual  poppy  plants  being,  on  an 
average,  1.166,  or  20.9  per  cent.  less.  Another  sample 
was  collected  in  some  fields  at  the  top  of  the  Chilterns, 
and  in  this  case  the  variability  was  1.770,  or  20.2  per 
cent,  greater  than  in  the  other  sample.  Moreover,  the 
*  "  Grammar  of  Science."  p.  387. 


BLASTOGENIC  VARIATIONS.  183 

mean  number  of  bands  was  also  greater,  it  being  10.04 
as  against  9.84.  In  a  third  sample  collected  from  still 
another  locality,  the  variability  was  1.455,  but  the  mean 
number  of  bands  was  only  8.77.  Supposing,  therefore, 
equal  numbers  of  specimens  had  been  collected  from  all 
three  localities  and  combined,  the  variability  would 
have  been  about  double  the  average  variability  of  the 
individual  groups  of  plants.  Supposing  samples  had 
been  collected  from  numerous  and  more  widely  sepa- 
rated localities,  so  as  to  get  a  representative  sample  of 
the  whole  species,  then  doubtless  the  variability  would 
have  been  much  greater  still.  Individual  variability 
may  therefore  be  only  slightly  smaller  than  local  racial 
variability,  but  it  is  very  much  smaller  than  specific 
variability. 

What  is  true  for  plants  is  true  also  for  animals. 
Supposing  that  in  the  case  of  the  middle  classes  of  Eng- 
lish society,  the  average  variability  of  the  stature  of  all 
the  offspring  is  only  about  10  per  cent,  more  than  that 
of  the  offspring  of  individual  parents,  then  it  is  clear 
that  if  we  were  to  include  also  representatives  of  the 
lower  and  of  the  upper  classes  in  our  sample,  the  aver- 
age variability  would  be  somewhat  greater,  perhaps  12 
per  cent.  If  we  were  to  include  representatives  in  due 
proportions  from  all  the  continental  nations,  then  the 
variability  might  be  25  per  cent,  or  more  in  excess,  and 
if  from  all  the  nations  of  the  world,  with  African  pyg- 
mies on  the  one  hand,  and  Patagonian  giants  on  the 
other,  then  it  might  be  50  per  cent,  greater,  or  even 
more. 

Asexual  Reproduction  in  Plants.  In  plants  asexual 
reproduction  is  very  much  more  common  than  in  ani- 


184  BLASTOGENIC   VAEIATIONS. 

mals,  and  though,  until  the  above-mentioned  memoir 
was  published,  there  was  practically  no  statistical  evi- 
dence as  to  the  range  of  variation  then  experienced,  as 
compared  with  that  found  in  sexually  reproduced  forms, 
there  was  available  the  common  knowledge  of  every 
horticulturist  and  nurseryman,  were  he  scientifically 
trained  or  otherwise.  Thus  it  had  been  thoroughly  well 
established  that  asexually  produced  forms,  i.  e.,  grafts, 
cuttings,  offsets,  and  tubers,  are  characterised  by  a  very 
much  greater  constancy  than  sexually  produced  forms, 
e.  g.,  seedlings.  For  concrete  instances  I  cannot  do  bet- 
ter than  quote  from  those  given  by  Mr.  Sedgwick  in  his 
recent  Presidential  Address  before  the  British  Asso- 
ciation.* For  example,  in  the  asexual  propagation  of 
the  potato  by  tubers,  the  plants,  be  they,  for  instance,  of 
the  Magnum  Bonum  variety,  give  rise  to  plants  exactly 
resembling  their  parent  in  foliage,  flowers,  and  tubers; 
if  they  be  of  the  Snowdrop  variety,  the  foliage,  flowers, 
habit,  and  tubers  are  also  similar,  and  are  totally  dif- 
ferent from  those  of  the  Magnum  Bonum.  "  In  this 
way  a  favourable  variety  of  potato  can  be  reproduced 
to  almost  any  extent  with  all  its  peculiarities  of  earli- 
ness  or  lateness,  pastiness  or  mealiness,  power  of  re- 
sisting disease  and  so  forth.  By  asexual  reproduction 
the  exact  facsimile  of  the  parent  may  always  be  ob- 
tained, provided  the  conditions  remain  the  same." 
Supposing,  on  the  other  hand,  one  tries  to  raise  Magnum 
Bonum  plants  from  seed,  in  all  probability  not  one  of 
the  seedlings  will  exactly  reproduce  the  parents;  they 
will  all  be  different,  both  in  properties  of  keeping,  re- 
sisting disease,  and  so  forth.  "  Again,  take  the  apple : 
*  Nature,  vol.  xl.  p.  502,  1899. 


BLASTOGENIC  VARIATIONS.  185 

if  you  sow  the  seed  of  the  Blenheim  Orange  and  raise 
young  apple  trees,  you  will  not  get  a  Blenheim  Orange. 
All  your  plants  will  be  different,  and  probably  not  one 
will  give  you  apples  with  the  peculiar  excellence  of 
the  parent.  If  you  want  to  propagate  your  Blen- 
heim Orange  and  increase  the  number  of  your 
trees,  you  must  proceed  by  grafting  or  by  striking 
cuttings." 

In  the  face  of  such  evidence  as  this,  it  seems  impos- 
sible for  Professor  Pearson  to  maintain  his  belief  that 
the  function  of  sex  in  evolution  "  is  not  the  production 
of  greater  variability."  At  the  same  time,  his  results 
above  quoted  show  the  incorrectness  of  the  view  some- 
times held,  that  variability  is  quite  insignificant  in 
asexually,  as  compared  with  that  in  sexually,  reproduced 
forms.  Statistically  measured,  it  is  only  10  to,  say,  50 
per  cent,  less,  though  when  this  amount  is  translated 
into  differences  of  foliage  and  flowers,  or  of  quality  of 
fruit,  it  seems  at  first  sight  much  more  considerable. 

Bud-Variation.  Considerable  variations  may  arise 
asexually  in  cases  of  so-called  bud-variation.  This 
term  was  used  by  Darwin  to  designate  the  sudden 
changes  in  structure  and  appearance  which  occasionally 
occur  in  the  flower-buds  or  leaf-buds  of  full-grown 
plants.  Such  changes  are  known  to  gardeners  as 
sports,  but,  as  we  have  already  seen,  this  term  is  now 
used  to  include  all  suddenly  arising  discontinuous  vari- 
ations. 

One  of  the  best  known  and  most  striking  instances  of 
bud-variation  is  that  of  the  nectarine,  which  occasion- 
ally appears  on  full-grown  peach  trees  which  have  pre- 


186  BLASTOGENIC   VARIATIONS. 

viously  borne  peaches  alone.  This  is  the  more  remark- 
able as  most  varieties  of  both  the  peach  and  the  nec- 
tarine reproduce  themselves  truly  by  seed.  Again, 
nectarine  stones  occasionally  yield  peach  trees,  and  a 
single  instance  is  recorded  of  a  full-grown  nectarine 
tree  bearing  perfect  peaches.*  Numerous  other  in- 
stances of  bud-variation  have  been  observed  in  the 
plum,  cherry,  vine,  gooseberry,  currant,  and  other 
fruits,  but  it  is  unnecessary  to  refer  to  these  here.  In 
flowering  plants,  also,  many  cases  have  been  recorded 
of  a  whole  plant,  or  a  single  branch  or  bud,  suddenly 
producing  flowers  different  from  the  proper  type  in 
colour,  form,  size,  or  other  character.  For  instance, 
a  Chrysanthemum,  raised  from  seed,  produced  by  bud- 
variation  six  distinct  varieties,  five  differing  in  colour, 
and  one  in  foliage.f  The  common  double  moss-rose 
probably  took  its  origin  from  the  Provence  rose  by 
bud-variation.  The  leaves  and  shoots  may  be  modi- 
fied by  bud-variation  as  well  as  the  flowers,  and  several 
varieties  of  trees  have  probably  originated  in  this 
manner. 

(      As  to  the  cause  of  bud-variation,  we  are  in  the  ma- 
jority of  cases  entirely  ignorant.     Darwin  attributes 
I  many  of  the  cases  to  reversion  to  characters  previously 
p/  present,  but  which  have  been  lost  for  a  longer  or  shorter 
\time.     Other  cases  he  attributes  to  the  plants  being  of 
crossed  parentage,  and  to  the  buds  reverting  to  one  of 
the  two  parent  forms.     There  are  still  many  cases  left, 
^however,  in  which  what  appear  to  be  absolutely  new 
characters  present  themselves.     These  can  only  be  at- 

*  "  Animals  and  Plants,"  1.  p.  362. 
f  "  Animals  and  Plants,"  i.  p.  440. 


BLASTOGENIC  VARIATIONS.  187 

tributed  to  so-called  spontaneous  variabilify.  Though 
in  individual  instances  it  may  be  difficult  or  impossible 
to  assign  any  reason  for  the  sudden  appearance  of  such 
"  spontaneous  variability,"  yet  various  observations  in- 
cline one  to  believe  that  it  is  probably,  after  all,  only  a 
special  instance  of  variations  due  to  changed  conditions 
of  life.  Thus  it  is  noticeable  that  all  plants  which  have 
yielded  bud-variations  have  likewise  varied  greatly  by 
seed.  They  seem,  in  fact,  to  possess  an  inherent  varia- 
bility. Again,  almost  all  plants  showing  bud-varia^ 
tion  have  been  highly  cultivated  for  long  periods,  in 
many  soils  and  under  different  climates.  On  the  other 
hand,  plants  living  under  their  natural  conditions  are 
very  rarely  subject  to  bud-variation.  In  some  in- 
stances, as  when  all  the  fruit  on  a  purple  plum  tree  sud- 
denly becomes  yellow,  or  all  the  fruit  on  a  double- 
flowered  almond  suddenly  becomes  peach-like,  we 
seem  to  perceive  a  direct  result  of  changed  condi- 
tions of  life;  but  more  often  than  not  we  are  com- 
pelled to  conclude  that  the  connection  is  only  an  indirect 
one.  t 

Darwin  points  out  *  that  it  is  "  a  singular  and  inex- 
plicable fact  that  when  plaUts  vary  by  buds,  the  varia- 
tions, though  they  occur  with  comparative  rarity,  are^ 
often,  or  even  generally,  strongly  pronounced."  In 
plants  raised  from  seed,  however,  the  variations  are  al- 
most infinitely  numerous,  but  their  differences  are  gen- 
erally slight.  Bud-variations  clearly  seem,  therefore, 
to  be  true  discontinuous  variations,  and  not  merely 
exaggerated  normal  variations.  As  to  the  ultimate 
cause  of  their  production,  we  are  as  completely  in  the 
*L.  c.,i.  p.  443.  + 


188  BLASTOGENIC  VARIATIONS. 

dark  as  we  were  for  the  analogous  phenomena  observed 
in  sexually  produced  forms.  We  can  only  conclude,  as 
we  did  then,  that  the  process  of  development  at  some 
point  takes  on  a  new  and  abnormal  departure,  the  direct 
or  indirect  result  of  changes  of  environmental  condi- 
tions. 


CHAPTER  VI. 

CERTAIN    LAWS    OF    VARIATION. 

Effect  of  environment  on  growth  diminishes  rapidly  from  time  of 
impregnation  onwards — Reaction  of  an  organism  to  environment 
dependent  on  nature  of  organism — Rapidly  diminishing  rate  of 
growth  in  man  and  in  the  guinea-pig  with  progress  in  develop- 
ment— Variability  also  diminishes  with  growth— Effect  on  growth 
once  produced,  probably  never  eradicated — Increased  variability 
of  sparrow  and  of  periwinkle  in  America — Relation  between  vari- 
ability and  want  of  adaptation  to  environment — Variability  of 
migratory  and  non-migratory  birds — Does  domestication  increase 
variability? 

BEFORE  entering  on  the  discussion  of  the  causes  of 
so-called  somatogenic  variations,  i.  e.,  of  acquired  char- 
acters, it  will  be  well  to  examine  at  some  little  length 
certain  more  or  less  general  laws  and  conditions  which 
control  their  acquisition  and  retention.  This  is  the 
more  necessary,  as  the  matter  has  received  such  very 
little  attention  hitherto.  It  seems  to  have  been  more 
or  less  tacitly  assumed  that  external  conditions  act 
equally  powerfully  at  all  periods  in  the  growth  of  a  de- 
veloping organism,  whilst  the  persistence  or  otherwise 
of  any  effect,  once  produced,  has  scarcely  been  debated 
at  all. 

In  what  way,  then,  does  a  developing  organism  react 
in  its  growth  to  the  conditions  of  its  surroundings?  It 
would  probably  be  concluded  that  any  given  change  of 
environmental  condition  would  produce  more  effect  in 


190  CERTAIN    LAWS    OF    VARIATION. 

the  earlier  stages  of  development  than  in  the  later,  but 
what  is  the  numerical  expression  of  this  difference? 
Such  an  expression  can  be  obtained  in  two  ways:  di- 
rectly, as  the  result  of  experiment ;  and  indirectly,  from 
certain  considerations  as  to  the  rate  of  growth  and  per- 
sistence of  variations. 

To  determine  the  effect  of  environment  on  growth, 
almost  any  organism  can  be  made  use  of,  but  it  would 
obviously  be  exceedingly  troublesome  and  laborious  to 
work  with  the  higher  organisms,  such  as  mammals.  In 
them  the  earliest  stages  of  embryonic  development 
would  be  especially  difficult  to  reach.  With  many  of 
the  lower  organisms,  however,  all  such  difficulties  are 
avoided,  and  an  inexhaustible  supply  of  material  can 
readily  be  obtained.  For  these  reasons  the  author  at- 
tempted to  investigate  the  question  at  issue  by  observa- 
tions on  the  larvae  of  sea-urchins.  The  method  of  ex- 
periment has  already  been  indicated  in  Chapter  IV., 
so  that  it  is  unnecessary  to  refer  to  it  again. 

The  object  in  view  was  to  determine  the  permanent 
effect  of  some  abnormal  environmental  condition,  act- 
ing at  various  periods  of  development,  on  the  size  of  the 
larvae.  It  was  found  that  their  growth  practically 
ceased  after  6  or  8  days,  and  hence  any  effect  then 
found  to  be  present  was  fixed  and  ineradicable,  so  far 
as  the  larval  stage  of  the  organism  was  concerned.  The 
most  convenient  environmental  condition  to  work  with 
proved  to  be  temperature,  for  it  is  easily  controlled, 
and  the  effect  produced  may  be  considerable.  Thus 
larvae  kept  at  10°  C.  during  growth  are  some  25  per 
cent,  smaller  than  those  kept  at  20°  C.* 
*Phil.  Trans.  1898,  B.  p.  481. 


CERTAIN  LAWS  OF  VARIATION. 


191 


To  determine  the  effect  of  temperature  acting  at  the 
time  of  impregnation,  portions  of  the  ova  and  sperma- 
tozoa were  shaken  from  the  ovaries  and  testes  in  small 
beakers  of  sea-water.  After  bringing  these  to  the  re- 
quired abnormal  temperature,  their  contents  were 
mixed,  and  the  mixed  solution  kept  at  the  same  tempera- 
ture for,  in  most  cases,  an  hour.  It  was  then  poured 
into  a  jar  holding  2  to  4  litres  of  sea-water  at  the  normal 
temperature.  The  ova,  now  fertilised,  were  allowed  to 
develop  under  as  constant  conditions  as  possible  for  8 
days,  and  the  larvae  were  then  killed  and  measured  in 
groups  of  50,  as  already  mentioned.  Other  ova,  kept 
at  the  time  of  impregnation  at  a  normal  instead  of  an 
abnormal  temperature,  were  allowed  to  develop  under 
otherwise  exactly  similar  conditions,  and  so  afforded 
"  control  "  or  "  normal  "  larvae,  against  which  the  effect 
produced  in  the  other  larvae  by  exposure  to  the  abnor- 
mal temperature  could  be  determined.  In  the  accom- 
panying table  the  results  obtained  in  the  various  obser- 
vations are  collected:  * 


*83* 

H> 

'ilig 

H  p  N  P  P 

P 

PERCENTAGE  DIMINUTION  PRODUCED 
IN  SIZE  OF  LARY^E. 

I! 

2  PH  •«  H  H 

3 

1  hour 

about  8° 
11    25.5° 

about  19° 

8.5,  1.8,  8.3,  0.0,  1.2,  2.5,  8.7,  2.5,  3.6,  4.2. 
3.3,  13.8.  4.7.  +  1.0,  5.1,  9.4,  6.0. 

4.1* 
5.9 

1  or  3  min. 

"      8° 

.3,  6.9,  3.6,  2.4. 

3.3 

"    25.5° 

" 

10.6,  1.9,  2.7. 

5.1 

10  seconds 

8°  or  25.5° 

2.4,  2.7,  +  .2,  1.9. 

1.7 

Here  we  see  that  exposure  for  an  hour  to  a  tempera- 
ture of  about  8°  C.  at  the  time  of  impregnation,  instead 

*  Vide  Phil.  Trans.  1895,  B.  p.  582,  and  Proc.  Roy.  Soc.,  vol.  xlvii. 
p.  85,  1900. 


192  CERTAIN  LAWS  OF  VARIATION. 

of  one  of  19°  C.,  produced,  in  ten  observations,  an  aver- 
age diminution  of  4.1  per  cent,  in  the  size  of  the  larvae. 
Temperatures  a  few  degrees  above  the  normal  acted 
even  more  unfavourably,  one  of  25.5°  producing,  in 
seven  observations,  an  average  diminution  of  5.9  per 
cent.  It  follows,  therefore,  that  at  the  time  of  their 
impregnation,  the  ova  are  most  extraordinarily  sensi- 
tive to  the  temperature  of  their  surroundings,  be  it  ab- 
normally high  or  abnormally  low.  Further  observa- 
tions showed  that  they  were  also  very  sensitive  to  an- 
other condition,  viz.,  salinity  of  the  water,  though  not 
to  the  same  extent  as  to  temperature.  It  seems  very 
probable,  therefore,  that  at  this  period  they  are  very 
sensitive  to  all  conditions  of  environment,  whatever 
their  nature. 

The  results  contained  in  the  lower  half  of  the  table 
are  even  more  remarkable  than  those  in  the  upper. 
Thus,  if  the  ova  were  kept  at  about  8°  or  at  25.5°  for 
only  one  to  three  minutes  after  the  mingling  of  the 
solutions  containing  the  ova  and  spermatozoa,  and  after 
this  short  period  were  poured  into  jars  of  water  at  nor- 
mal temperature,  an  average  diminution  in  size  of  re- 
spectively 3.3  and  5.1  per  cent,  was  effected!  The  ob- 
servations made  are  not  very  numerous  or  regular,  and 
hence  not  much  importance  can  be  attached  to  the  actual 
figures,  but  one  is  justified  in  concluding  that  the  effect 
produced  is  not  so  very  much  smaller  than  when  the 
period  of  exposure  to  the  abnormal  temperature  <was  an 
hour.  Now  during  a  minute's  time  little  more  than  the 
impregnation  of  the  ovum  would  be  accomplished. 
The  processes  connected  with  the  fusion  of  the  male 
and  female  pronuclei  and  the  commencement  of  seg- 


CERTAIN  LAWS  OF  VARIATION.  193 

mentation  can  scarcely  have  begun;  hence  it  must  be 
the  temperature  of  the  ovum  and  the  spermatozoon 
during  the  act  of  impregnation  which  is  of  such  con- 
siderable influence  on  the  subsequently  developing 
larvae,  the  effect  perhaps  depending  on  the  vibratile 
energy  with  which  the  spermatozoon  strikes  and  enters 
the  ovum.  In  the  last  line  of  the  above  table  are  re- 
corded a  few  experiments  in  which  the  time  of  exposure 
to  the  abnormal  temperature  was  reduced  to  ten  sec- 
onds. The  average  effect  then  produced  was  only  1.7 
per  cent.,  or  less  than  half  as  much  as  in  the  other  ob- 
servations. Presumably,  therefore,  the  time  was  in- 
sufficient for  all  the  ova  to  become  impregnated  at  the 
abnormal  temperature. 

It  may  perhaps  be  thought  that  this  extraordinary 
sensitiveness  of  the  ova  to  temperature  at  the  time  of 
impregnation  is  true  only  for  the  particular  species  of 
sea-urchin  worked  with,  viz.,  Strongylocentrotus  lividus, 
and  that  one  is  in  no  way  justified  in  regarding  it  as  a 
general  phenomenon.  How  far  this  criticism  is  true  or 
not,  future  experiment  alone  can  show,  but  the  sensi- 
tiveness was  at  least  demonstrated  in  one  other  case, 
viz.,  the  ova  of  the  sea-urchin  Sphcer echinus  granularis. 
Exposure  of  the  ova  for  one  hour  to  a  temperature  of 
about  11°  produced  in  four  experiments  respectively 
7.7,  1.1,  4.6  and  2.6  per  cent,  diminution  in  the  size  of 
the  eight-days'  larvae.  Exposure  to  a  temperature  of 
27°  was  much  less  effectual,  it  producing  in  two  experi- 
ments respectively  2.5  and  .9  per  cent,  diminution.  In 
two  other  experiments,  exposure  to  this  temperature 
for  a  minute  instead  of  an  hour  produced  respectively 
4.3  and  .9  per  cent,  diminution.  In  explanation  of 


194  CERTAIN  LAWS  OF  VARIATION. 

these  slighter  effects  it  should  be  mentioned  that  this 
organism  throughout  its  growth  was  found  to  be  dis- 
tinctly less  reactive  to  its  environment  than  was 
Strongylocentrotus. 

In  order  to  determine  the  effect  of  exposure  to  ab- 
normal temperatures  during  the  later  stages  of  devel- 
opment, all  the  Strongylocentrotus  ova  were  kept  for 
the  first  hour  during  impregnation  at  the  same  tempera- 
ture, and  were  then  divided  up  into  two  portions.  One 
portion  was  poured  into  a  jar  of  water  at  a  normal 
temperature,  and  the  larvae  obtained  therefrom  used  as 
control  larvae,  whilst  the  other  was  poured  into  water 
previously  brought  to  an  abnormal  temperature.  After 
a  few  hours,  the  contents  of  this  jar  were  vigorously 
stirred,  and  some  of  them  poured  off  into  a  smaller  jar. 
This  was  then  transferred  to  the  tank  of  running  water 
containing  the  control  jar,  and  kept  there  during  the 
remainder  of  larval  develpoment.  A  few  hours  later 
another  portion  was  withdrawn  from  the  abnormal  tem- 
perature jar,  and  similarly  treated.  The  kind  of  result 
thereby  obtained  may  be  gathered  from  the  following 
table: 

SIZE.  PER  CENT.  DIMINUTION  IN  DURING 

SIZE  PER  HOUR. 

Normal  larvae  (22. 5°) ,  100.0 

1-6  hrs.  at  12°,             93.61  1.28  1-6  hrs. 

1-10  hrs.        "                92.37  .31  6-10  hrs. 

1-21  hrs.        "                90.09  .21  10-21  hrs. 

Here  the  size  of  the  "  normal "  larvae,  grown  through- 
out at  a  temperature  of  22.5°,  is  taken  as  100. 
Larvae  obtained  from  ova  kept  at  a  temperature  of 
about  12°  from  the  end  of  the  first  hour  after  impreg- 
nation to  the  end  of  the  sixth  hour  were  found  to  be 


CERTAIN  LAWS  OF  VARIATION. 


195 


6.4  per  cent,  smaller  than  these,  or  were,  on  an  average, 
diminished  in  size  1.28  per  cent,  for  each  hour's  expos- 
ure. An  exposure  of  the  ova  for  a  further  four  hours 
produced  only  1.24  per  cent,  more  diminution  in  the 
size  of  the  larvae,  or  .31  per  cent,  per  hour,  whilst  still 
eleven  hours  more  produced  an  additional  2.28  per  cent, 
diminution,  or  .21  per  cent,  per  hour.  Taking  means  of 
the  various  hours  of  exposure,  one  can  therefore  say 
that  the  effect  produced  during  hour  3J  was  1.28  per 
cent.,  or  about  four  times  greater  than  that  produced 
during  hour  8,  and  six  times  greater  than  that  in  hour 
15J.  That  is  to  say,  the  effect  produced  very  rapidly 
diminished  with  progress  in  development. 


NORMAL  LARVAE  AT  ABOUT  20°: 

NORMAL  LARVAE  AT  13°: 

ABNORMAL  AT  8°. 

ABNORMAL  AT  22°. 

Time  of  ex- 
posure in 
hours. 

Mean    time 
in  hoars. 

Per  cent,  dimi- 
nution in  size 
per  hour. 

Time  of  ex- 
posure in 
hours. 

Mean    time 
in  hours. 

Per  cent,  i  n  - 
crease  in  size 
per  hour. 

0-  1 

.5 

4.14 

1-8 

4.5 

1.08 

1-  6 

3.5 

1.28 

1-11 

6 

.0 

1-  9 

5 

1.17 

8-19 

13.5 

.37 

6-10 

8 

.31 

1-28 

14.5 

.40 

10-21 

15.5 

.21 

19-43 

31 

•0 

1-84 

42.5 

.13 

28-71 

49.5 

.125 

19-192 

105.5 

.0 

84-192 

138 

.022 

Other  observations  were  made  at  a  colder  season  of 
the  year,  and  in  these  the  normal  larvae  were  kept  at 
about  13°,  and  the  "  abnormal "  at  about  22°.  An  in- 
crease in  the  size  of  the  larvae  was  of  course  produced 
thereby,  and  here  again  the  effect  was  very  much 
greater  for  exposure  during  the  earlier  hours  of  devel- 


196  CERTAIN  LAWS  OF  VARIATION. 

opment  than  during  the  later.  In  the  foregoing 
table  are  collected  all  the  observations  made  by  both 
methods,  the  mean  effect  above  mentioned  as  being 
produced  by  an  hour's  exposure  to  a  temperature  of  8° 
at  the  time  of  impregnation  being  also  included.  We 
accordingly  see  that  the  unfavourable  influence  of  the 
cold  steadily  diminishes  from  the  time  of  impregnation 
up  to  the  15th  hour;  whilst  in  the  other  series  of  obser- 
vations the  favourable  influence  of  the  warmth  also 
diminishes  rapidly,  though  not  so  regularly.  In  one  or 
two  cases,  for  some  unknown  reason,  apparently  no 
effect  was  produced;  but  allowing  for  these  by  taking 
means  of  the  values  obtained  at  more  or  less  similar 
hours,  we  still  find  a  rapid  and  fairly  regular  decrease. 
Thus,  at  about  the  120th  hour  the  average  effect  pro- 
duced was  only  about  a  fiftieth  of  that  at  the  fifth  hour. 
In  still  a  third  series  of  observations,  some  of  the  ova 
were  kept  for  various  periods  at  about  26°  C.  Dur- 
ing the  first  few  hours  of  development  this  tempera- 
ture had  an  extremely  unfavourable  action,  produc- 
ing a  diminution  in  size  varying  from  20.76  to  7.36  per 
cent.  Still  further  exposure  to  it,  however,  produced 
a  favourable  effect  on  growth,  though  this  was  never 
sufficient  entirely  to  counteract  the  previous  adverse  in- 
fluence. The  results  obtained  in  two  of  the  experi- 
ments made  are  given  in  the  table  below.  Here  we 
see  that  from  the  end  of  the  first  to  the  end 
of  the  fourth  hour  average  diminutions  of  3.89  and 
6.45  per  cent,  per  hour  were  produced.  During  the 
next  four  hours  this  adverse  effect  dwindled  down  to 
nothing,  or  almost  nothing,  and  in  the  next  four  hours 
a  distinctly  positive  effect  set  in.  This  probably  con- 


CERTAIN  LAWS  OF  VARIATION. 


197 


tinned  for  the  remainder  of  the  period  of  growth.  If 
we  neglect  the  transition  periods  in  these  two  series, 
we  see  that  the  effect  of  the  abnormal  temperature,  be 
it  negative  or  positive,  rapidly  diminishes  with  progress 
in  development.  In  the  17th  hour  it  is  not  a  fifteenth 
and  in  the  83rd  hour  not  a  four  hundredth  as  great  as 
in  hour  2^.  In  two  other  series  of  observations  a 
similar  kind  of  result  was  obtained;  so  taking  together 
all  the  series  of  observations  made,  by  all  the  methods, 
one  is  therefore  justified  in  concluding  that  the  effect 
of  temperature  on  the  growth  of  the  developing  organ- 
ism diminishes  rapidly  and  regularly  from  the  time  of 
impregnation  onwards. 


PER  CENT. 

SIZE  OP 

VAR.  PER 

DURING 

SIZE  OF 

VAR.  PER 

LARVAE. 

HOUR. 

LARVAE. 

HOUR. 

Normal  larvae  (at 

23.5°  and  24.2°) 

100.00 

100.00 

-4  hours  at  26° 

88.33 

-3.89 

1—4  hrs. 

80.64 

—6.45 

—7^5  or  8  hrs.  " 

88.90 

4-  .14 

4-7%  hrs. 

79.24 

—  .40 

-11  or  12  hrs.  " 
-22  hrs. 
—  144  hrs. 

94.56 
99.31 
98.43 

4-1.42 
4-  .47 
—  .007 

22—  144  hrs.' 

80.21 
84.25 

87.27 

4-  .28 
4-  .20 
4-  .025 

These  experiments  in  which  the  same  condition  of 
environment  at  first  produced  an  adverse  effect,  and 
then  a  favourable  one,  illustrate  a  very  important  prin- 
ciple; one  which  was  recognised  long  ago  by  Darwin, 
and  also  by  Weismann.  They  show  that  the  reaction 
of  an  organism  to  its  environment  depends  on  the 
nature  of  that  organism.  Thus  with  respect  to  the  di- 
rect action  of  conditions  of  life,  "  We  must  bear  in 
mind,"  says  Darwin,  "  that  there  are  two  factors ; 
namely,  the  nature  of  the  organism  and  the  nature  of 
the  conditions.  The  former  seems  to  be  much  the 


198  CERTAIN  LAWS  OF  VARIATION. 

more  important;  for  nearly  similar  variations  some- 
times arise  under,  as  far  as  we  can  judge,  dissimilar 
conditions;  and  on  the  other  hand,  dissimilar  variations 
arise  under  conditions  which  appear  to  be  nearly  uni- 
form." *  The  reasons  of  these  differences  of  reaction 
are,  as  a  rule,  quite  unknown  to  us,  but  in  the  present 
instance  a  satisfactory  explanation  was  arrived  at. 
Thus,  by  heating  up  portions  of  the  ova  to  various  tem- 
peratures at  various  periods  of  development,  it  was 
found  that  the  temperature  at  which  they  were  killed 
was  by  no  means  constant.  It  increased  regularly  with 
progress  in  development,  so  that  it  was  about  12°  C. 
higher  for  full-grown  plutei  than  for  ova  at  the  time  of 
impregnation. 

STAGE  OF  TIME  AFTER  DEATH 

DEVELOPMENT.  IMPREGNATION.  TEMPERATURE. 

Strongylocentrotus  ova,  28.5° 

Semi-blastulse,  4  hours  33.5° 

Blastulae  and  semi-gastrulse,  12  hours  36.5° 

Plutei  and  semi-plutei,  28  hours  39.5° 

Plutei,  6  days  40.3° 

Supposing,  therefore,  a  temperature  of  33.5°  is  fatal  to 
four  hours'  blastulse,  then  all  temperatures  a  few  de- 
grees lower  than  this  must  be  unfavourable  to  their 
growth.  Lower  temperatures  still  we  know  to  be  highly 
favourable.  Now  we  saw  that  a  temperature  of  26° — 
i.  e.,  one  3°  or  4°  below  the  death  temperature — was 
very  unfavourable  to  the  growth  of  ova  shortly  after 
impregnation;  so  to  produce  an  equally  unfavourable 
effect  on  four  hours'  blastulse,  we  should  probably  need 
a  temperature  of  30°  or  so,  and  on  12  hours'  blastulse, 
one  of  33°.  Or  conversely,  the  temperature  of  26,° 
*  •'  Origin  of  Species,"  Ed.  6,  p.  6. 


CERTAIN  LAWS  OF  VARIATION.  199 

which  proved  itself  so  favourable  to  the  growth  of  12 
hours'  blastulse,  would  correspond  to  a  temperature  of 
23°  acting  on  4  hours'  blastulse,  and  one  of  about  19° 
acting  on  ova  shortly  after  impregnation. 

To  return  to  our  conclusion  as  to  the  diminishing 
effect  of  temperature  with  development,  it  seems  rea- 
sonable to  imagine  that  what  is  true  for  one  environ- 
mental condition  is  probably  true  for  others  also.  Ob- 
servations were  in  fact  made  to  test  the  effects  of  a 
change  of  salinity,  and  they  also  proved  the  ova  to  be 
very  much  more  sensitive  in  the  earlier  stages  of  their 
growth  than  in  the  later.*  Here  again,  however,  a 
double  effect  was  produced,  adverse  for  the  first 
few  hours  of  exposure,  and  favourable  for  the  later 
ones. 

Due  reflection  will,  I  believe,  incline  one  to  infer  that 
what  is  true  for  Echinoid  larvae  is  true  for  most  multi- 
cellular  organisms.  In  fact,  it  would  seem  to  be  a  law 
of  general  application  that  the  permanent  effect  of  en- 
vironment on  the  growth  of  a  developing  organism 
diminishes  rapidly  and  regularly  'from  the  time  of  im- 
pregnation onwards. 

It  is  curious  that  this  principle,  enunciated  by  the 
author  in  1900,  should  have  been  laid  down  by  De 
Vries,t  only  a  few  months  later,  as  the  result  of  his  ob- 
servations on  plants.  Thus,  judging  from  the  effects 
of  nutrition  (manuring,  planting  out,  good  light,  and 
watering),  he  concluded  that:  (1)  The  younger  a  plant 
is,  so  much  the  greater  is  the  influence  of  external  con- 
ditions on  its  variability.  (2)  The  nutrition  of  the 

*  Vide  Proc.  Roy.  Soc.,  vol.  Ixvii.  p.  97,  1900. 
f"  Die  Mutationstheorie,"  p.  373. 


200  CERTAIN  LAWS  OF  VARIATION. 

seed,  when  developing  on  the  maternal  plant,  has — at 
least  very  often — a  greater  influence  on  the  variability 
than  nutrition  during  its  germination  and  later  growth. 

De  Vries  does  not  quote  any  concrete  instance  in  sup- 
port of  the  first  of  these  conclusions,  but  in  proof  of  the 
second,  he  adduces  some  observations  on  CEnothera  La- 
marckiana.*  On  sowing  some  seed  of  this  plant  in 
well-manured  ground,  he  found  that  the  seed  capsules 
of  the  plants  obtained  were  slightly  smaller  than  those 
from  seeds  grown  in  unmanured  ground  (the  mean 
lengths  being  respectively  25.2  and  27.2  mm.).  The 
seeds  of  three  of  the  manured  plants,  when  sown  next 
year,  yielded  plants  with  seed  capsules,  on  an  average, 
29.9  to  33.4  mm.  in  length.  This  considerable  in- 
crease seems  to  have  been  almost  entirely  due  to 
the  manure  treatment  received  by  the  plants  from 
which  the  seeds  had  been  derived,  and  in  compari- 
son with  it,  artificial  selection  was  not  nearly  so 
effective.  Thus  the  (well-manured)  plants  arising 
from  seeds  taken  from  long  seed  capsules  (32.6  to  43.0 
mm.  in  length)  had  themselves  capsules  of  only  31.6  to 
33.4  mm.,  whilst  those  from  the  seeds  of  short  seed  cap- 
sules (15.6  to  23.4  mm.),  had  capsules  of  24.2  to  29.9 
mm. 

Doubtless  the  numerical  measure  of  the  effect  of  en- 
vironment on  growth  varies  enormously  in  different 
organisms,  and  the  extent  of  its  variation  can  be  deter- 
mined only  by  experiment.  Fortunately,  however, 
numerous  data  which  have  been  collected  as  to  the  rate 
of  growth  in  certain  animals,  more  especially  in  man  and 
the  guinea-pig,  can  be  readily  applied  to  the  partial 
*Loc.  cit.,  p.  383. 


CERTAIN  LAWS  OF  VARIATION.  201 

elucidation  of  the  law  in  question,  though  they  are  in- 
sufficient to  afford  an  absolute  criterion  of  its  extent. 
Thus  it  is  obvious  that  the  reaction  of  a  growing  organ- 
ism to  its  environment  must  depend  on  its  rate  of 
growth  at  the  time.  Supposing  that  in  the  earliest 
stages  of  development  it  doubles  its  weight  within  a 
day,  whilst  in  the  later  stages  only  within  a  month,  then 
an  unfavourable  environmental  condition,  acting  for  a 
given  time  during  the  early  period,  may  conceivably 
produce  thirty  times  more  retardation  of  growth  than 
when  acting  during  the  later  period.  To  what  degree 
retardations  of  growth  produced  at  various  stages  of 
development  persist  to  the  adult  stage  is  another  ques- 
tion, and  reference  will  be  made  to  it  later  on. 

In  the  case  of  man,  the  data  relative  to  the  embryonic 
rate  of  growth  are  not  very  reliable,  but  they  are  quite 
sufficiently  so  for  our  purpose.  A  considerable  number 
of  them  have  been  collected  by  Preyer.*  His  has  fur- 
nished data  as  to  the  length  of  embryos  during  the  first 
few  weeks  of  development,  and  Hennig  has  given  a  table 
of  measurements  for  the  1st  to  the  10th  month,  these 
being  averages  derived  from  a  hundred  observations. 
As  Minot  points  out,t  however,  these  data  are  inexact, 
for  in  the  early  stages  only  the  head  and  trunk  were 
measured,  and  in  the  later  stages  head,  trunk,  and  legs. 
In  human  embryos  weight  is  a  better  criterion  of  de- 
velopment than  length,  but  as  in  the  observations  of 
His  only  the  length  was  determined,  it  is  necessary,  for 
purposes  of  comparison,  to  keep  to  this  standard 
throughout.  In  order  to  obtain  readily  comparable 

*  "  Specielle  Physiologic  des  Embryo,"  p.  500,  Leipsig,  1885. 
f  "  Human  Embryology,"  p.  336,  New  York,  1892. 


202 


CERTAIN  LAWS  OF  VARIATION. 


values,  the  times  required  by  the  embryo  at  the  various 
stages  of  development  in  order  to  double  its  length  have 
been  calculated: 


PERIOD  OF  DE- 

LENGTH DOUBLED 

PERIOD  OP  DE- 

LENGTH DOUI 

VELOPMENT. 

DURING 

VELOPMENT. 

DURING 

2£—  2|  weeks 

6.6davs 

4—5  months 

36.  7  da 

2|  —  31.     •« 

9.5 

5—6       ' 

78.2    • 

3^-4 

7.8 

6—7       ' 

146.3    • 

1—2  months 

11.6 

7—8       ' 

202.5    • 

2—3 

26.2 

8—9       ' 

306.1     ' 

3—4 

29.6 

9—10     ' 

518.5    ' 

From  this  table  we  see  that  the  rate  of  growth  of  the 
human  embryo  diminishes  steadily,  with  one  slight  ex- 
ception, from  the  earliest  to  the  latest  stages.  So 
enormous  is  the  diminution  in  the  growth  that  in  the 
last  month  of  pregnancy  it  is  only  about  an  eightieth 
part  of  that  in  the  third  week.  In  the  still  earlier 
stages  it  seems  valid  to  assume  that  the  relative  rate  of 
growth  increases  at  the  same  proportionate  rate  as  in 
the  later  ones;  and,  arguing  from  the  more  or  less 
known  volumes  of  the  ovum  at  the  time  of  concep- 
tion, and  of  the  embryo  in  its  third  week  of  growth,  it 
has  been  roughly  calculated  that  the  first  doubling  in 
diameter  of  the  fertilised  ovum  requires  less  than  three 
days,  or  about  1-2  00th  part  of  the  time  in  the  last 
month  of  pregnancy. 

With  regard  to  post-embryonic  growth  rate,  but  little 
need  be  said.  A  large  number  of  observations  have 
been  made  on  the  length  of  children  at  birth,  and  the 
mean  of  all  the  values  for  both  sexes  is  approximately 
50.0  cm.  From  Bowditch's  tables  of  the  statures  of 
Boston  school  children,*  it  is  found  that  the  mean 
*  Report  of  Massachusetts  State  Board  of  Health,  p.  275,  1877. 


CERTAIN  LAWS  OF  VARIATION.  203 

height  of  girls  and  boys  between  their  5th  and  6th 
years  is  105.2  cm.,  or  only  just  over  double  the  birth 
length.  The  rate  of  growth  during  this  period  is  there- 
fore only  about  1-2 80th  part  of  that  in  the  third  week 
of  embryonic  growth.  Between  the  5th  and  10th 
years  it  is  only  about  l-900th  part,  and  between  the 
15th  and  19th  years  only  l-2400th  part.  Compared 
with  the  calculated  rate  of  growth  during  the  first  day 
of  development,  the  rate  during  this  last  period  is  about 
5000  times  smaller. 

As  regards  another  mammal,  the  guinea-pig,  a  very 
complete  series  of  observations  has  been  made  by 
Minot,*  though  unfortunately  these  extend  only  to  the 
post-embryonic  stages.  Minot  has  calculated  the  daily 
percentage  increase  in  weight  of  his  animals,  and  he 
found  that  this  diminished  steadily  from  the  fourth  day 
after  birth  onwards.  During  the  first  week  or  two  it 
was  over  5  per  cent,  per  diem;  at  the  end  of  the  second 
month  about  1.3  per  cent.,  and  after  a  year  about  .1 
per  cent.  Minot  seems  to  have  been  the  first  observer 
to  recognise  the  importance  of  calculating  the  relative 
rate  of  growth,  as  distinguished  from  the  absolute  in- 
crements of  weight.  He  considers  there  can  be  little 
doubt  that  there  is  a  progressive  loss  of  growth  power 
at  least  in  all  mammals,  and  perhaps  in  all  living  beings. 

That  the  rate  of  growth  in  the  earliest  stages  of  de- 
velopment is  enormously  more  rapid  than  in  the  later 
ones  may  therefore  be  regarded  as  proven.  And  it 
seems  to  me  that  it  is  probably  a  general  rule,  with 
doubtless  some  exceptions,  that  the  effect  of  environ- 
ment on  the  growth  of  an  organism  depends  chiefly  on 
*J.  Physiol.,  xii.  p.  97. 


204  CERTAIN  LAWS  OF  VARIATION. 

the  rate  of  growth  at  the  time.  To  what  extent,  how- 
ever, does  a  retardation  or  acceleration  of  growth  pro- 
duced at  any  particular  period  of  development  persist 
to  the  adult  stage?  It  is  quite  conceivable  that  such 
an  effect,  produced  in  the  earliest  stages,  may  be  en- 
tirely compensated  for  by  a  subsequent  variation  of  the 
growth  in  a  reverse  direction,  and  leave  no  trace  be- 
hind. On  the  other  hand,  it  is  conceivable  that  it  may 
persist  unchanged  and  undiminished.  In  all  proba- 
bility, however,  it  does  neither  the  one  or  the  other. 
The  initial  effect  gradually  diminishes  with  progress  of 
development,  but  never  entirely  disappears.  An  effect, 
once  produced,  can  never  be  wholly  eradicated.  Upon 
this  point  some  of  the  data  given  by  Minot  are  again 
most  pertinent.  He  calculated  the  arithmetical  mean 
error  in  the  weights  of  more  than  a  hundred  guinea- 
pigs,  which  were  bred  under  as  normal  and  favourable 
conditions  as  possible,  and  weighed  at  short  intervals 
from  the  day  of  birth  up  to  the  end  of  their  second 
year.  During  the  first  fortnight  of  growth,  the  indi- 
vidual weights  were  on  an  average  found  to  vary  from 
the  mean  weight  by  ±19.0  per  cent,  in  male  guinea- 
pigs,  and  ±17.2  per  cent,  in  females.  Between  the 
70th  and  140th  days  this  variability  had  sunk  to 
±12.7  per  cent,  for  males,  and  ±14.7  per  cent,  for  fe- 
males. During  the  145th  to  215th  days,  it  fell  still 
further  to  ±7.5  per  cent,  for  males,  and  ±13.0  per 
cent,  for  females.  At  later  periods  it  remained  con- 
stant in  females,  but  increased  again  somewhat  in 
males.  Roughly  speaking,  therefore,  one  may  say  that 
with  progress  in  development  from  the  earliest  to  the 
latest  stages,  the  variability  becomes  nearly  halved. 


CERTAIN  LAWS  OF  VARIATION.  205 

Quite  recently,  Professor  Pearson  *  has  shown  that 
in  man  also  the  diminution  in  variability  with  growth  is 
very  marked.  Working  on  measurements  of  1000 
male  and  1000  female  new-born  babies,  he  obtained 
the  following  values  for  the  weight  and  length: 

MEAN  MEAN  PER  CENT.  VARIABILITY 

WEIGHT.  LENGTH.  WEIGHT.  LENGTH. 

Females,  7.073  Ibs.  20.124  in.  14.228  5.849 

Males,      7.301  Ibs.  20.503  in.  15.664  6.500 

Here  we  see  that  the  male  infant  at  birth  is  slightly 
heavier  and  slightly  longer  than  the  female,  and  also 
that  its  relative  variability  (error  of  mean  square  ex- 
pressed as  a  percentage  on  the  mean  value)  is  distinctly 
greater.  On  working  out  similar  data  for  160  female 
students  (mostly  from  19  to  25  years  of  age),  and  for 
1000  male  students  of  the  same  age,  Pearson  obtained 
the  following  values: 

MEAN  MEAN  PER  CENT.  VARIABILITY 

WEIGHT.  LENGTH.  WEIGHT.  LENGTH. 

Females,    125.605  Ibs.         63.883  in.  11.170  3.696 

Males,        152. 784  Ibs.         68.863  in.  10.830  3.662 

The  woman  is  now  slightly  more  variable  than  the 
man,  both  in  weight  and  length,  but  the  variability  of 
both  is  very  much  less  than  at  birth.  It  is,  in  fact,  on 
an  average,  26.4  per  cent,  less  as  regards  weight,  and 
40.4  per  cent,  less  as  regards  length. 

As  variability  undergoes  such  marked  diminution 
with  age  both  in  man  and  the  guinea-pig,  we  may  ven- 
ture to  agree  with  Minot  that  probably  this  diminution 
occurs  in  the  growth  of  all  mammals,  and  in  fact  that 

*Proc.  Roy.  Soc.,  Ixvi.  p.  23,  1900. 


206  CERTAIN  LAWS  OF  VARIATION. 

variability  of  all  characteristics  may  similarly  diminish. 
Should  this  contention  be  justified  by  subsequent  re- 
search, as  seems  very  probable,  then  it  might  be  formu- 
lated as  a  definite  law.  This  could  be  worded  as  fol- 
lows: "The  variability  of  a  developing  organism 
diminishes  regularly  with  its  growth."  Doubtless 
there  are  many  exceptions  or  partial  exceptions  to  this 
law.  For  instance,  Bowditch  *  has  shown  that  in  the 
case  of  human  stature,  there  is  a  distinct  increase  of 
variability  just  before,  and  at  the  time  of,  puberty  in 
boys,  and  a  slight  one  in  girls,  followed  by  a  decrease. 
Weldon  f  has  found  that  the  variability  of  the  frontal 
breadths  of  young  crabs  somewhat  increases  with 
growth,  and  then  diminishes.  Still  these  exceptions 
are  not  sufficient  to  upset  the  general  validity  of  the 
law.  As  additional  evidence  in  favour  of  it,  may  be 
cited  the  observations  of  Bumpus  $  on  the  variability  of 
the  periwinkle.  Calculating  from  measurements  made 
on  13,000  shells  from  different  sources,  it  is  found  that 
the  variability  in  the  ventricosity  or  relative  breadth  of 
the  small  and  medium-sized  shells  is  distinctly  greater 
than  that  of  the  larger  (and  therefore  on  an  average 
older)  shells.  Again,  I  myself  have  found  that  the 
variability  in  the  size  of  Strongylocentrotus  larvae  di- 
minishes steadily  from  the  fifth  day  of  growth  on- 
wards^ 

If  our  contentions  as  to  rate  of  growth  and  variability 
be  admitted,  it  follows  that  the  variability  of  embryonic 

*  Report  of  Massachusetts  State  Board  of  Health,  p.  275,  1877. 

fProc.  Roy.  Soc.,  Ivii.  p.  367. 

j  Zool.  Bulletin,  vol.  i.  p.  247. 

§Phil.  Trans.  1895,  B.  p.  617  (and  confirmed  by  subsequent  ob- 
servations). 


CERTAIN  LAWS  OF  VARIATION.  207 

animals  should  be  very  great  indeed.  Upon  this  point 
there  is,  unfortunately,  very  little  evidence,  though  the 
general  opinion  of  embryologists  supports  the  view. 
However  Fischel  *  has  made  several  different  measure- 
ments on  104  embryos  of  the  duck,  and  he  classified  his 
results  according  to  the  number  of  vertebrae  they  con- 
tained. He  found  that  the  variability  was  very  great, 
and  also  that  it  diminished  considerably  with  growth, 
but  the  values  are  too  irregular  for  brief  quotation. 

As  the  variability  of  guinea-pigs  becomes  nearly 
halved  during  post-embryonic  development,  it  fol- 
lows that  any  retardation  or  acceleration  of  growth 
produced  in  the  earliest  stages  must  also  become  nearly 
halved,  or  perhaps  still  more  diminished.  A  certain 
amount  of  variation  must  have  been  produced  in  the 
growth  of  the  animals  by  variation  in  their  environ- 
mental conditions  during  development,  though  prob- 
ably, as  these  conditions  appear  to  have  been  kept  as 
constant  and  as  favourable  as  possible,  this  was  not 
very  large.  One  may  perhaps  assume,  therefore,  that 
if  absolutely  constant  environmental  conditions  had 
been  maintained,  the  variability,  when  growth  had 
ceased,  would  not  have  been  reduced  to  very  much  less 
than  half  its  original  amount.  Now  by  the  end  of  the 
first  year  the  rate  of  growth  had  diminished  to  about  a 
fiftieth  its  amount  at  birth,  and  hence,  in  order  to  pro- 
duce equal  permanent  effects  upon  the  growth,  the 
period  of  exposure  of  an  animal  to  a  changed  environ- 
mental condition  shortly  after  birth  would  probably 
need  to  be  only  about  a  twenty-fifth  part  of  the  time 
required  a  year  later.  Again,  to  revert  to  the  data  ob- 
*Morph.  Jahrb.,  xxiv.  p.  369. 


208  CERTAIN  LAWS  OF  VARIATION 

tained  in  the  case  of  man,  we  saw  that  between  the  15th 
and  19th  years  the  rate  of  growth  was  only  about  1- 
2400th  part  of  that  obtaining  in  the  third  week  of 
foetal  existence.  Now  granted  that  any  effect  pro- 
duced by  a  changed  environmental  condition  at  this 
early  period  dwindled  down  to  even  a  tenth  of  its  origi- 
nal amount  by  the  time  the  adult  stage  had  been 
reached,  yet  the  organism,  as  far  as  the  permanent 
effect  of  environment  is  concerned,  would  still  be  240 
times  more  sensitive  in  the  one  case  than  in  the  other. 

Striking  evidence  in  support  of  this  conclusion  that 
retardations  of  growth,  once  produced,  are  never  totally 
eradicated,  has  been  recorded  by  Galton.*  A  friend 
of  his  took  monthly  measurements  of  the  circumference 
of  his  children's  heads  during  the  first  few  years  of 
their  lives,  and  by  plotting  them  out  on  paper,  obtained 
curves  of  growth.  These  curves  were  regular  on  the 
whole,  but  each  of  them  showed  occasional  halts,  which 
were  found  to  coincide  in  point  of  time  with  the  various 
infantile  diseases  the  children  had  experienced.  These 
illnesses  had  therefore  arrested  the  development,  and 
this  "  had  not  been  made  up  for  by  after  growth."  In 
fact  "  disease  had  drawn  largely  upon  the  capital,  and 
not  only  on  the  income  of  their  constitutions." 

A  few  of  Minot's  data  afford  similar  evidence.  In 
one  of  his  guinea-pigs,  No.  34,  the  increase  in  weight 
was  quite  normal  at  first,  but  then  absolutely  ceased 
from  the  90th  to  the  110th  day.  After  this  it  con- 
tinued normally  as  before,  but  the  animal  never  suc- 
ceeded in  recovering  its  loss,  and  after  two  years 
weighed  only  about  two-thirds  as  much  as  the  other 
*"  Inquiries  into  Human  Faculty,"  p.  235. 


CERTAIN  LAWS  OF  VARIATION.  209 

animals.  One  or  two  of  the  other  observations  show  a 
somewhat  similar,  though  not  nearly  so  marked, 
result. 

If  our  theoretical  proof  of  the  extreme  sensitiveness 
of  an  organism  to  its  environment  during  the  earliest 
stages  of  development  is  held  to  be  as  valid  for  Mam- 
mals as  the  experimental  proof  showed  it  to  be  for 
Echinoderms,  then  we  are  met  with  a  fact  of  some  prac- 
tical importance.  We  are  afforded  a  scientific  support 
of  the  widely  held  belief  in  the  special  influence  of  the 
condition  of  the  mother  at  the  time  of  conception  upon 
the  subsequently  developing  offspring.  Should  such 
conception  occur  when  the  blood  and  tissues  of  the 
mother  are  tainted  by  products  of  metabolism,  the  re- 
sult of  excessive  fatigue,  or  by  alcohol,  or  by  the 
products  of  disease,  there  seems  no  reason  why  a  con- 
siderable and  lasting  adverse  influence  should  not  be 
produced  on  the  growth  of  the  offspring.  Still  again, 
we  must  conclude  that  the  health  of  the  mother  dur- 
ing pregnancy,  and  of  the  offspring  during  their  early 
years,  is  of  much  more  importance  in  its  permanent 
effects  on  the  physique  and  constitution  than  the 
health  in  later  years. 

Effect  of  Other  Conditions  on  Variability.  We  have 
seen  that  the  variability  of  a  developing  organism  is  far 
from  remaining  constant,  but  what  is  the  condition  of 
affairs  in  an  adult  organism  ?  At  the  present  day  it  has 
generally  been  more  or  less  tacitly  assumed  that  as  a 
rule  variability  remains  practically  constant,  and  that 
external  conditions,  though  they  may  affect  the  aver- 
age values  of  the  characters  of  an  organism,  do  not  in- 
fluence the  range  of  variation  of  these  characters 


210  CERTAIN  LAWS  OF  VARIATION. 

around  their  mean.  In  his  Presidential  Address, 
already  referred  to,  Mr.  Sedgwick  has,  however,  come 
to  a  somewhat  novel  conclusion  as  to  the  change  of 
variability  with  evolution.  He  points  out  that  through 
the  action  of  Natural  Selection  certain  variations 
will  be  gradually  eliminated,  and  the  organisms 
will  become  more  and  more  closely  adapted  to  their 
surroundings.  The  variability  of  the  species  will 
therefore  diminish.  Thus  breeders  have  found  that 
"  continued  selection  tends  to  produce  a  greater  and 
greater  purity  of  stock,  so  that  if  selective  breeding  is 
carried  too  far,  variation  almost  entirely  ceases."  It 
follows,  therefore,  that  "  variation  must  have  been 
much  greater  in  past  times  than  it  is  now.  In  fact,  it 
must  have  been  progressively  greater  the  farther  we  go 
back  from  the  present  time." 

This  specious  and  apparently  straightforward  argu- 
ment cannot  be  accepted,  however,  as  it  is  built  up  on 
false  premises.  It  is  by  no  means  true  that  by  selective 
breeding  we  can  reduce  variability  almost  to  nil.  Pro- 
fessor Pearson*  has  calculated  that,  in  the  human  race, 
if  the  parents  be  selected,  then  the  variability  of  the 
offspring  will  be,  on  an  average,  only  9.5  per  cent,  less 
than  that  of  the  whole  race;  whilst  if  the  grandparents 
and  any  number  of  more  remote  ancestors  be  selected 
as  well,  the  variability  will  never  become  more  than 
10.56  per  cent,  smaller  than  that  of  the  race.  In  fact, 
the  variability  of  offspring,  as  compared  with  that  of 
parents,  depends  directly  upon  the  correlation  between 
them.  If  the  coefficient  of  correlation  be  r,  then  the 
variability  of  the  offspring  of  selected  parents  will  be 
*  "  Grammar  of  Science,"  p.  485. 


CERTAIN  LAWS  OF  VARIATION.  211 

VT— r2.*  For  instance,  we  have  seen  that  in  the  case 
of  man — and  likewise  also  in  other  organisms — the  co- 
efficient of  correlation  between  parent  and  offspring 
is  .3,  and  between  mid-parent  and  offspring  .424. 
The  variability  of  all  the  children  of  parents  of  any 
given  stature  will  therefore  be  Vl-ri)90— .9539,  or  95 
per  cent,  of  that  of  the  race,  and  the  variability  of  the 
children  of  mid-parents  of  any  given  stature  will  be 
90.55  per  cent,  of  that  of  the  race.  Again,  Professor 
Pearson  and  his  co-workers  t  have  found  that  in  the 
series  of  measurements  upon  plants  examined  by  them, 
there  is  no  relation  whatever  between  the  variability  of 
the  species  and  the  intensity  of  homotyposis.  Thus 
on  splitting  up  all  the  22  series  of  measurements  into 
two  groups,  the  mean  variability  of  the  first  group  (as 
expressed  by  the  coefficient  of  variation)  was  22.95  per 
cent.,  and  of  the  second,  14.28  per  cent.,  or  not  two- 
thirds  as  much.  Nevertheless  the  mean  homotypic 
correlation  (or  degree  of  similarity  between  the  undif- 
ferentiated  like  organs  of  an  individual,  as  compared 
with  organs  of  other  individuals  of  the  same  race)  was 
.456  in  the  first  group,  and  .458  in  the  second;  i.  e., 
was  practically  the  same.  "  Hence  there  seems,"  says 
Professor  Pearson,  "  so  far  as  our  researches  go,  no 
ground  for  asserting  that  increased  intensity  of  heredity 
means  decreased  intensity  of  variation,  and  vice  versa." 
To  return  to  Mr.  Sedgwick's  views,  if  rigid  artificial 
selection  can  only  reduce  variability  by  11  per  cent., 
then  obviously  Natural  Selection  can  scarcely  have  any 

*L.  c.,  p.  397  and  p.  472. 
fPhil.  Trans.  A.  p.  275,  1901. 


212  CERTAIN  LAWS  OF  VARIATION. 

appreciable  influence  upon  it.  A  more  potent  factor  in 
diminishing  variability,  or  at  least  in  keeping  it  from 
increasing,  is  probably  to  be  found  in  the  greater  fer- 
tility of  the  most  commonly  occurring  forms  of  a  race. 
As  already  mentioned  in  Chapter  III.,  this  phenomenon 
has  thus  far  been  shown  to  exist  only  in  certain  plants, 
and  in  a  Hydromedusa,  but  the  presumption  is  that  it  is 
of  much  wider  application.  Our  knowledge  of  the 
matter  is,  however,  insufficient  to  warrant  an  extended 
discussion. 

An  apparent  instance  of  increase  of  variability  ac- 
companying cessation  of  the  action  of  Natural  Selection 
has  been  furnished  by  Bumpus,*  in  the  case  of  the 
common  sparrow.  These  birds  were  first  introduced 
from  Europe  into  America  in  1850,  and  they  have 
spread  so  rapidly,  owing  to  their  having  abundant  food 
and  practically  no  natural  enemies,  that  the  continent  is 
now  inundated  with  them.  The  Natural  Selection, 
which  has  kept  the  birds  when  in  their  native  habitat 
more  or  less  subservient  to  the  regiilations  imposed  by 
competing  life,  seems  in  this  case  to  be  largely  sus- 
pended. According  to  Bumpus  nearly  all  the  young 
birds  reach  maturity;  variations  in  colour  and  struct- 
ure, unless  most  extreme,  apparently  not  being  disad- 
vantageous to  their  possessor.  In  order  to  compare  the 
variability  of  this  American  race  of  sparrows  with  the 
native  European  one,  it  was  found  convenient  to  make 
measurements  of  the  eggs,  rather  than  of  the  birds 
themselves,  as  they  are  so  much  less  readily  procurable. 
Eight  hundred  and  sixty-eight  American  and  868  Eng- 
lish eggs  were  compared  with  respect  to  length, 
*  Biol.  Lectures,  Wood's  Holl,  1896-97,  also  Science,  v.  p.  423. 


CERTAIN  LAWS  OF  VARIATION.  213 

breadth,  general  shape,  and  colour.  The  American 
eggs  were  slightly  more  variable  in  length,  they  vary- 
ing from  18  to  26  mm.  instead  of  from  18.5  to  25  mm., 
and  also  were  reduced  in  their  average  length  from  22 
to  21  mm.  As  Davenport  has  shown,*  however,  the 
mean  deviation  or  arithmetical  mean  error,  which  is  a 
better  index  of  the  variability,  is  somewhat  smaller  in 
the  American  eggs  than  in  the  English;  but  the  arith- 
metical error  of  the  variations  in  the  relation  of  breadth 
of  egg  to  length  is  slightly  greater  in  the  American  eggs 
than  in  the  English  (in  the  proportion  of  .73  to  .70). 
In  order  to  compare  the  variations  in  the  shape  of  the 
eggs,  such  as  conical,  ellipsoidal,  pear  and  lemon  shapes, 
Bumpus  requested  a  disinterested  person  to  select  from 
the  1736  mixed  eggs  the  hundred  eggs  which  appeared 
to  him  most  variable.  The  American  eggs  were  known 
by  a  secret  mark,  and  it  was  found  that  81  of  the 
selected  eggs  were  American,  and  only^!9  English.  By 
a  somewhat  similar  process  of  selection  with  reference 
to  extremes  of  colour  marking,  it  was  found  that  82  of 
the  100  selected  eggs  were  American,  and  18  English. 
There  can  be  little  doubt,  therefore,  that  American  eggs 
are  considerably  more  variable,  both  in  shape  and 
colour,  than  English  eggs. 

The  conclusions  to  be  drawn  from  this  most  interest- 
ing series  of  observations,  however,  are  probably  not 
those  suggested  by  the  author.  In  the  first  place,  as 
Davenport  has  pointed  out,  they  afford  presumptive  evi- 
dence that  the  American  eggs,  or  the  birds  which  pro- 
duced them,  were  subjected  to  a  distinct  selective 
process.  Thus  the  curve  of  distribution  of  the  length 
*  L'Annee  Biologique,  1897,  p.  496. 


214  CERTAIN  LAWS  OF  VARIATION. 

of  these  eggs  is  distinctly  asymmetrical,  due,  one  may 
imagine,  to  the  weeding  out  of  a  larger  proportion  of 
the  birds  producing  the  smaller  eggs.  The  curve  of 
distribution  of  the  English  eggs  is  practically  sym- 
metrical, so  that  selection  in  them  probably  removed 
extreme  individuals  to  an  equal  extent  in  either  direc- 
tion. It  should  be  borne  in  mind,  however,  that  the 
skewness  of  the  curve  of  distribution  of  the  American 
eggs  may,  after  all,  be  due  only  to  Reproductive  Selec- 
tion, i.  e.y  to  a  greater  fertility  of  the  birds  producing 
the  larger  eggs. 

To  what,  then,  are  the  diminished  size  of  the  eggs  and 
their  increased  variability  due?  Probably  they  are 
nothing  more  than  a  direct  response  to  the  change  in  the 
environmental  conditions.  These  conditions,  though, 
by  virtue  of  the  absence  of  enemies  and  other  checks, 
more  suitable  to  increase  in  the  numbers  of  the  spar- 
rows, are  probably  more  variable,  and,  on  the  whole,  not 
so  suitable  to  individual  growth.  Thus  in  England  the 
sparrows  have  been  subjected  to  a  very  keen  competi- 
tion for  a  very  long  time,  and  so  have  become  closely 
adapted  to  their  surroundings.  In  other  words,  their 
environment  is  as  favourable  as  it  can  be  for  their 
maximum  growth,  and  for  a  minimum  of  variability. 

Another  series  of  observations  made  by  Bumpus  *  to 
prove  his  views  may  from  their  intrinsic  interest  be 
cited  also.  They  concern  the  variations  of  the  peri- 
winkle, LiHorina  littorea.  This  mollusc  appears  to 
have  been  first  carried  to  American  shores  about  1850, 
and  since  then  it  has  spread  with  great  rapidity  along 
the  New  England  coast.  For  statistical  purposes,  1000 
*Zo51.  Bulletin,  vol.  i.  p.  247. 


CERTAIN  LAWS  OF  VARIATION.  215 

specimens  of  the  shell  were  obtained  from  each  of  ten 
different  localities  in  America,  and  from  three  in  Great 
Britain.  The  variation  was  estimated  by  determining 
the  ratio  of  breadth  to  length  in  each  shell,  or  the  ven- 
tricosity,  and  expressing  the  result  as  a  percentage  of 
the  one  on  the  other.  The  extreme  variations  recorded 
were  respectively  80  per  cent,  and  104  per  cent.,  or  a 
difference  of  24.  As  a  measure  of  the  variability 
Bumpus  took  the  difference  in  the  extreme  values  ex- 
hibited by  each  sample  of  shells,  and  he  found  that  the 
amplitude  of  variation  of  the  American  shells  was  in 
every  case  in  excess  of  that  of  the  British  shells.  Thus 
it  was  respectively  18,  19,  20,  19,  17,  20,  20,  18,  18, 
and  20  in  the  American  samples,  and  15,  14,  and  12  in 
the  British.  Obviously,  however,  this  is  not  a  very 
good  method  of  estimating  variability,  as  it  is  more  or 
less  a  matter  of  chance  if  specimens  of  shells  exhibiting 
the  most  extreme  variations  happen  to  occur  in  any 
given  sample  of  1000.  Thus  in  two-thirds  of  all  of 
Bumpus'  series,  the  extreme  variations  were  repre- 
sented by  only  a  single  specimen.  Fortunately  the  true 
value  of  these  extensive  series  of  observations  has  not 
been  lost,  as  Duncker  *  has  taken  the  trouble  to  work 
through  them  and  calculate  the  variability  by  an  exact 
method.  The  following  are  the  values  obtained  by 
him  for  the  mean  ventricosity,  and  its  variability  (as 
expressed  by  the  error  of  mean  square)  in  each 
sample. 

From  the  table  we  gather  that  the  average  ventri- 
cosity of  the  American  shells  is  slightly  greater  than 
that  of  the  British  (91.01  as  against  89.78,  or  1.4  per 
*Biol.  Centralblatt.,  vol.  xviii.  p.  569,  1898. 


216  CERTAIN  LAWS  OF  VARIATION. 

cent,  greater).  The  variability  is  in  every  case  con- 
siderably greater  for  the  American  shells,  as  Bumpus 
had  concluded.  The  average  variability  of  all  the 
American  samples  is  2.765,  or  18.2  per  cent,  larger 
than  the  average  for  the  British  (2.34). 

LOCALITY.  MEAN.  VARIABILITY. 

I.    f  Tenby,  Wales,      .        .        .        .90.96  2.38 

II.   \  South  Kincardineshire,  Scotland,    87.85  2.34 

III.  [Humber  District,          .        .        .90.53  2.30 

IV.  St.  Croix  River,  Maine,       .        .91.26  2.70 
V.      Casco  Bay,  Maine,       .        .»        .92.53  2.67 

VI.  Beverly,  Mass.,    .        .  .  .90.65  2.76 

VII.  Nahant,  Mass.,     .-^    .  .  .92.19  3.03 

VIII.  Plymouth,  Mass.,         .  ,    .  .90.09  2.48 

IX.  Seaconnet,  R.  L,          .  .  .89.72  2.86 

X.  Newport,  R.  1 89.17  2.62 

XI.  Bristol ,  R.  I.  (shingle),  Y  .    90 . 77  2 . 75 

XII.  Bristol,  R.  L  (sand),     .  .  .91.07  2.83 

XIII.  Warren  River,  R.  L     .  .  .92.69  2.95 

Bumpus  found  that  in  other  respects  also  the  Ameri- 
can shells  are  more  variable  than  the  British.  Thus 
they  show  greater  variations  of  ventricosity  in  the 
course  of  growth;  shells  of  a  given  length  exhibit  a 
greater  range  of  variation  in  weight;  also  their  colour 
markings  are  more  variable.  Hence  there  can  be  no 
doubt  that  the  general  variability  of  the  organism  has 
increased.  As  in  the  case  of  the  sparrow's  egg, 
Bumpus  attributes  this  to  its  emancipation  from  many 
of  the  restraining  influences  of  Natural  Selection;  but 
it  seems  to  me  a  simpler  explanation,  as  was  suggested 
in  the  case  of  the  eggs,  to  regard  the  effects  produced  as 
largely,  if  not  entirely,  the  direct  result  of  want  of 
adaptation  to  the  changed  and  perhaps  more  variable 
conditions  of  life. 


CERTAIN  LAWS  OF  VARIATION.  217 

That  want  of  adaptation  should,  as  a  rule,  lead  to  in- 
creased variability  may  not  seem  at  first  sight  obvious, 
but  a  little  reflection  will  show,  I  think,  that  it  is  only 
what  one  ought  on  a  priori  grounds  to  expect.  Thus 
supposing  a  group  of  organisms  is  subjected  to  unfav- 
ourable environmental  conditions,  so  that  they  are  all 
decreased  in  size  by,  on  an  average,  say  10  per  cent. 
We  know  by  experience  that  all  organisms  are  not 
equally  affected  by  changed  conditions.  Some  of  them 
are  more  resistant  than  the  average,  and  others  less. 
Consequently,  supposing  the  group  of  organisms  had 
originally  varied  between  such  and  such  limits,  then  on 
reduction  of  their  average  size  by  this  10  per  cent., 
some  of  the  small  individuals,  and  likewise  some  of  the 
large  ones,  will  be  diminished  by  perhaps  12  per  cent, 
in  size,  and  others  by  only  8  per  cent.,  and  hence  the 
range  of  variation  of  the  whole  group  of  organisms  will 
be  widened. 

Actual  proof  of  this  contention  has  been  obtained  by 
the  author  in  the  case  of  sea-urchin  larvae.  Thus,  in 
experiments  of  one  kind  and  another,  some  412  series  of 
measurements,  each  on  50  individuals,  have  been  made 
on  the  body  length  of  Strongylocentrotus  larvae  after  6 
or  8  days'  growth  under  various  favourable  and  unfav- 
ourable conditions.  The  variability  of  each  series  was 
determined  by  a  method  it  is  unnecessary  to  mention  in 
detail  here,*  that  of  the  whole  of  the  larvae  being  taken 
as  100.  All  the  series  of  larvae  grown  under  so-called 
"  normal "  conditions  (i.  e.,  in  jars  of  sea-water  kept  at 
as  constant  a  temperature  as  possible)  and  of  other 
larvae  which  did  not  vary  from  the  normal  by  more  than 
*  Vide  Phil.  Trans.,  1895,  B.  p.  617.  ^ 


218  CERTAIN  LAWS  OF  VARIATION. 

±1.9  per  cent.,  were  put  into  one  group,  and  a  mean 
taken  of  their  "  variability "  numbers.  The  larvae 
which,  owing  to  a  more  favourable  environment,  were 
larger  than  the  normal,  and  which  differed  from  it  by 
from  +  2  to  +  4.9  per  cent.,  were  similarly  grouped 
together.  Still  other  groups  were  formed  of  those  dif- 
fering by  from  '+  5  to  +  9.9  per  cent,  and  by  +  10 
per  cent,  and  more.  Like  groups  were  made  of  larvae 
which  adverse  environmental  conditions  had  rendered 
smaller  than  the  normal.  The  following  are  the  mean 
results : 

VARIATION  IN  BODT  LENGTH  OP  NUMBER  OF  SERIES     VARIABILITY 

LARVAE  FROM  NORMAL.  MEASUREMENTS. 

-f- 10  per  cent,  and  over,  .        .       '.        .  26  89 . 6 

5  to -f  9. 9  per  cent 30  968 

«-j-  4. 9  per  cent.,  ...        .  51  96.4 

±  1.9  per  cent.,  .        .        .        .170  96.8 

—  2  to  —  4.9  percent.,  .        .        .        .  63  100.8 
-  5  to  -  9.9  per  cent.,  ....  43  104.4 

—  10  per  cent,  and  over,  ....  29  113.2 

Here  we  see  that,  with  one  slight  exception,  the  varia- 
bility becomes  steadily  greater  and  greater  as  the  en- 
vironment becomes  more  and  more  unfavourable,  the 
extreme  value  in  one  direction  being  26.3  per  cent, 
greater  than  that  in  the  other.  This  experimental  re- 
sult, bearing  out  as  it  does  our  theoretical  conclusion, 
seems  to  justify  one  in  assuming  the  existence  of  a  defi- 
nite law  of  variability.  This  may  be  worded  as  fol- 
lows: "  An  organism  varies  least  when  it  is  best  adapted 
to  its  surroundings,  so  that  the  less  it  is  adapted,  the 
more  variable  does  it  become."  Extended  research 
alone  can  teach  us  how  general  may  be  the  application 
of  this  law,  and  afford  adequate  numerical  estimates  as 


CERTAIN  LAWS  OF  VARIATION.  219 

to  the  relation  between  want  of  adaptation  and  varia- 
bility in  different  organisms.  Without  doubt,  how- 
ever, these  must  vary  enormously. 

It  may  be  thought  that  this  law  is  a  mere  reproduc- 
tion, in  different  words,  of  one  of  the  numerous  causes 
of  variability  discussed  by  Darwin.  I  do  not  think  that 
this  is  the  case,  however,  for  though  he  states  that  varia- 
bility of  every  kind  is  due  to  changed  conditions  of  life, 
such  as  climate,  food,  and  excess  of  nutriment,  he  does 
not  suggest  that  the  real  factor  is  want  of  adaptation 
between  the  organisms  and  their  surroundings.  On 
the  contrary,  he  endeavours  to  show  that  the  amount  of 
modification  which  animals  and  plants  have  undergone 
does  not  correspond  with  the  degree  to  which  they  have 
been  subjected  to  changed  circumstances. 

From  the  conclusions  at  which  we  have  arrived,  it 
follows  that  increased  variability  of  environment  leads 
to  increased  variability  of  the  organisms  subjected  to  it. 
A  proof  that  this  is  the  case  has  been  afforded  by  Mont- 
gomery,* who  has  studied  the  variation  in  migratory 
and  non-migratory  species  of  North  American  birds. 
From  data  collected  by  Eidgway  in  his  "  Manual  of 
North  American  Birds,"  he  calculated  the  percentage 
amount  of  variation  (according  as  it  was  under  1  per 
cent.,  between  1  and  1.5  per  cent.,  between  1.5  and  2 
per  cent.,  or  over  2  per  cent.)  in  the  culmen  of  the  bill, 
wing,  tarsus,  and  tail  of  the  species  and  sub-species  of 
fifty-six  families.  His  results  show  "  that  migratory 
species  evince  a  greater  amount  of  individual  variation 
than  do  non-migratory  species ;  and  species  which  under- 
take extensive  migrations,  a  greater  amount  than 
*J.  Morph.,  1896,  p.  25, 


220  CERTAIN  LAWS  OF  VARIATION. 

species  which  make  migrations  of  less  magnitude." 
This  is  in  conformity  with  the  rule  that  species  inhabit- 
ing more  extensive  and  diversified  areas  show  more  in- 
dividual variation  than  those  with  small  or  insular 
breeding  areas,  and  also  with  another  rule,  "  that 
species  with  geographical  races  evince  a  greater  amount 
of  individual  variation  than  do  species  which  are  not 
divided  into  such  races,  provided  that  the  breeding  area 
is  approximately  equal  in  extent  and  diversification  in 
both  cases."  Again,  it  was  found  that  males  generally 
exhibit  a  greater  amount  of  individual  variation  than 
females  of  the  same  species,  the  males  being  most  vari- 
able in  60.4  per  cent,  of  the  273  cases  investigated,  and 
the  females  most  variable  in  39.6  per  cent,  of  them. 

From  these  facts  Montgomery  deduces  what  he  con- 
siders to  be  a  very  important  conclusion,  viz.,  that  "  in- 
dividual variation  is  greater  in  amount  in  those  species 
which  we  must  consider  under  the  influence  of  a  con- 
tinuous process  of  development  than  in  those  species 
which  we  must  consider  as  being  influenced  by  no 
process  of  development  at  all."  Though  this  view 
seems  highly  probable,  yet  it  appears  to  me  that  the 
data  adduced  do  not  in  any  way  prove  it.  Because  a 
bird  occupies  an  extensive  breeding  area,  or  is  migra- 
tory, it  does  not  follow  that  it  is  undergoing  develop- 
ment either  progressive  or  retrogressive.  The  only  de- 
duction which  seems  to  me  permissible  is  the  one  above 
mentioned,  one  which  has,  indeed,  been  drawn  by  Mont- 
gomery himself  in  the  following  words :  "  The  amount 
of  individual  variation  stands  in  a  direct  ratio  to  the 
degree  of  complexity  of  the  environmental  forces  which 
influence  the  organism," 


CERTAIN  LAWS  OF  VARIATION.  221 

One  of  the  few  conditions  which  have  been  generally 
held  to  lead  to  increased  variability  is  that  of  Domesti- 
cation. Darwin  *  laid  considerable  stress  on  the  fact 
that  "  with  extremely  few  exceptions  all  animals  and 
plants  which  have  been  long  domesticated  have  varied 
greatly."  This  increased  variability  he  attributed 
partly  to  the  conditions  of  life  being  less  uniform,  and 
to  a  lesser  degree  to  the  effects  of  excess  of  food.  He 
concluded  that  organic  beings  must  be  exposed  to  the 
new  conditions  for  several  generations  before  much  in- 
crease of  variability  is  observed,  and  hence  it  seemed  as 
if  the  changed  conditions  of  life  acted  not  only  directly 
on  the  organism  as  a  whole,  but  also  indirectly  through 
the  reproductive  system. 

In  the  first  place  it  is  probable  that  Darwin  consider- 
ably overestimated  the  variability  of  domesticated  or- 
ganisms, as  compared  with  that  of  undomesticated  ones. 
Thus  we  have  already  seen  how  widely  many  naturally 
occurring  organisms  may  vary,  and  Schwalbe  has  even 
denied  altogether  that  domestication  produces  any  in- 
crease of  variation.  Thus  he  states  f  that  Pfitzner  has 
found  that  parts  of  the  skeleton  of  the  fore  and  hind 
feet  of  wild  animals  vary  just  as  much  as  the  corre- 
sponding parts  in  man.  The  size  and  the  indices  of  the 
skull  of  the  otter,  as  determined  by  examining  over  200 
skulls  derived  from  a  limited  district  in  Alsace,  were 
found  by  Schwalbe  to  vary  just  as  much  as  the  corre- 
sponding measurements  in  the  most  widely  separated 
human  races.  Again,  Kohlbriigge  has  shown  that  in  all 


*  "  Animals  and  Plants,"  ii.  p.  401. 

f  Anatomischer  Anzeiger.  Verhandhmgen,  xii.  p.  2,  1898. 


222  CERTAIN  LAWS  OP  VARIATION. 

the  species  of  Primates  examined,  the  muscles  were 
quite  as  variable  as  in  man. 

Though  Schwalbe's  views  may  be  correct  as  regards 
some  domesticated  races,  they  obviously  cannot  apply 
to  all.  For  instance,  domesticated  pigeons,  dogs,  and 
horses  vary  between  very  much  wider  limits  than  any 
naturally  occurring  species.  But  it  is  obvious  that 
such  extreme  variations  as  these  owe  their  origin  chiefly 
to  careful  artificial  selection,  extending  over  very  large 
numbers  of  generations.  When  no  such  selection  is 
practised,  as  in  some  of  the  domesticated  animals  kept 
by  savages,  then  it  is  highly  probable  that  the  varia- 
bility is  no  greater  than  in  allied  feral  species.  Any 
slight  increase  of  variability  actually  present  may 
doubtless  be  attributed  to  the  direct  action  of  the 
changed  conditions  of  life. 

Darwin's  conclusion  that  increased  variability  only 
shows  itself  after  several  generations  of  domestication, 
if  true  as  it  stands,  is  difficult  to  account  for  in  any 
other  way  than  by  supposing  that  the  reproductive  sys- 
tem is  affected.  But  it  seems  to  me  distinctly  probable 
that  much,  if  not  all,  of  the  increased  variability  may 
be  ascribed  merely  to  the  careful  selection  of  the  most 
aberrant  individuals  in  each  generation,  and  their 
assortive  mating.  In  such  a  case  the  variability  of  the 
whole  group  would  obviously  increase  from  generation 
to  generation. 


CHAPTEE  VII. 

THE    EFFECT    OF    TEMPERATURE    AND    OF    LIGHT. 

Variations  and  modifications — Effect  of  temperature  on  growth  of 
frog — Optimum  temperature  of  growth  in  plants — Effect  of  tem- 
perature on  size  of  sea-urchin  larvae,  of  Lepidoptera,  and  of  Mol- 
lusca — Seasonal  dimorphism  in  certain  Lepidoptera  in  its  rela- 
tion to  temperature — Temperature  differences  giving  rise  also  to 
local  races,  sports,  and  phylogenetic  forms — Critical  period  of 
reaction  to  temperature — Effect  of  Arctic  climate  on  coat  of 
mammals — Effect  of  darkness  and  of  light  on  growth  of  plants — 
Effect  of  sunlight  and  of  diffused  light — How  far  does  pigmenta- 
tion of  animals  depend  on  exposure  to  light? — Cave  animals — 
Illumination  of  under  surface  of  flounder — Effect  of  light  and  of 
darkness  on  Molluscs — Variable  protective  resemblance  in  the 
frog,  in  fish,  and  in  larvae  and  pupae  of  certain  Lepidoptera. 

HAVING  discussed  blastogenic  or  germinal  variations 
as  fully  as  our  very  limited  knowledge  of  the  subject 
permits,  we  will  now  proceed  to  enquire  into  the  other 
great  class  of  variations,  namely,  those  produced  by  the 
action  of  the  environmental  conditions  on  the  soma  or 
body  tissues.  These  were  termed  by  Weismann  somato- 
genic  variations,  but  Lloyd  Morgan,  Mark  Baldwin,  and 
others,  with  a  view  to  distinguishing  them  more  point- 
edly from  the  former  class  of  variations,  have  referred 
to  them  as  somatic  modifications,  or  more  simply  as 
modifications.  The  term  "  variations "  they  reserve 
exclusively  for  blastogenic  variations.  Though  doubt- 
less there  is  much  to  be  said  for  this  system  of  classify- 
ing variations,  yet  there  is  also  something  against  it. 
It  is  frequently  difficult  or  impossible  to  decide  whether 


224  THE  EFFECT  OF  TEMPERATURE 

any  given  variation  is  of  blastogenic  or  somatogenic 
origin;  frequently,  indeed,  it  is  partly  the  one  and 
partly  the  other.  Hence  the  term  "  variation  "  is  con- 
venient for  general  use,  when  one  wishes  to  imply 
nothing  as  to  the  origin  of  the  observed  condition, 
whilst  the  narrower  meaning  may  be  applied  to  it  when 
it  is  mentioned  in  connection  with  the  recognised  and 
closely  defined  term  "  modification." 

In  our  discussion  of  somatic  modifications,  it  is  found 
most  convenient  to  classify  them  according  to  the 
agencies  which  have  produced  them,  and  not  according 
to  the  effects  produced.  Those  observations  in  which 
there  appears  to  be  a  clear  and  direct  relation  between 
cause  and  effect,  between  particular  environment  and 
particular  modification,  will  first  be  mentioned,  and  the 
mixed  and  often  indirect  effects  of  more  complex  con- 
ditions, such  as  climate  and  general  environment, 
studied  later. 

Temperature.  Of  all  the  environmental  conditions, 
temperature  is  probably  one  of  the  most  unequivocal  in 
its  direct  production  of  variations.  That  is  to  say,  in 
the  majority  of  cases  it  acts  directly  on  all  the  cells 
composing  the  tissues,  promoting  or  retarding  their 
growth,  and  producing  a  permanent  effect  on  the  organ- 
ism as  a  whole.  Of  course  it  does  not  necessarily  fol- 
low that,  because  the  rate  of  growth  is  altered,  a  per- 
manent effect  is  produced  on  the  absolute  size  of  the 
organism.  Yet  in  the  light  of  the  observations  de- 
scribed in  the  last  chapter,  we  are  justified  in  assuming 
that  such  is  as  a  rule  the  case  in  animals.  For  plants  it 
will  probably  be  generally  admitted,  if  the  difference 
in  the  size  of  trees  and  plants  grown  in  the  tropics  and 


AND  OF  LIGHT. 


in  temperate  climates,  or  in  temperate  and  arctic  ones, 
be  borne  in  mind. 

Some  striking  observations  on  the  effects  of  tempera- 
ture on  growth  were  made  by  Higginbottom  some  fifty 
years  ago.*  Ova  of  the  frog  were  allowed  to  develop 
under  otherwise  similar  conditions  at  the  temperatures 
15.6°,  13.3°,  11.7°,  and  10.5°  C.  The  tadpoles  were 
observed  to  leave  the  ova  respectively  9,  14,  20,  and  20 
days  after  the  beginning  of  the  experiment,  whilst 
frogs  were  fully  developed  after  respectively  72,  160, 
170,  and  234  days.  Thus  a  difference  of  5°  C.  more 
than  trebled  the  period  of  development. 


AMBLYSTOMA  TIGRINUM. 

RANA    VIRESCENS. 

s 

jjb 

*| 

i 

fa 

o  o 

n 

|o| 

2  «  a 

11 

?! 

||  | 

n'3 

11 

<x>  g_o 

S  <» 

*& 

Kg 

«J 

«1 

!• 

^*Jr 

4°C. 

288  hours 

2 

4°C. 

471  hours 

A 

8° 

210 

4 

8.75° 

192 

1 

9.5° 

139.2 

5 

12.12' 

126 

2 

13° 

96 

2 

16° 

60 

3 

14° 

90 

2 

22° 

27.5 

2 

18° 

60 

3 

24» 

25.5 

2 

22° 

40 

6 

26P 

21.5 

1 

Within  the  last  few  years,  a  considerable  number  of 
observations  have  been  made.  Thus  Lillie  and  Knowl- 
ton  f  experimented  with  the  ova  of  Amblystoma  tigri- 
num  and  of  the  frog  Rana  virescens.  The  time  of  de- 
velopment from  the  first  or  second  cleavage  to  the  last 


*Phil.  Trans.  1850,  p.  431. 


Zo61.  Bulletin,  vol.  i.  p.  179. 


226 


THE  EFFECT  OF  TEMPERATURE 


stage  of  disappearance  of  the  yolk  plug  was  determined, 
these  being  the  most  sharply  marked  periods.  The 
temperatures  varied  between  4°  and  26°  C.,  several  ob- 
servations being  made  at  each  temperature,  and  means 
taken. 

From  tbe  above  table  we  may  gather  that  at  the 


24023022021020°19018°17°16015014°13012''ll'>1009'>  8°  7°  6°  5°  4°  3°  2°  1° 

I?IG.  22. — Effect  of  temperature  on  growth  of  tadpole. 

highest  temperatures  employed  the  rate  of  develop- 
ment was  respectively  7.2  and  21.9  times  more  rapid 
than  at  the  lowest. 

In  the  accompanying  figure  are  reproduced  the  re- 


AND  OF  LIGHT.  227 

suits  obtained  by  O.  Hertwig  *  upon  the  ova  of  Mana 
fusca.  The  curves  represent  the  number  of  days  re- 
quired by  the  ova,  kept  at  different  temperatures,  to 
reach  certain  definite  stages.  The  ordinates  indicate 
the  time  in  days  after  fertilisation,  and  the  abscissae  the 
temperature.  For  the  lowest  curve  the  stage  to  be 
reached  was  that  of  a  gastrula  with  the  blastopore  clos- 
ing in,  and  we  gather  from  this  curve  that  the  time  re- 
quired at  a  temperature  of  1°  C.  was  23  days;  at  a  tem- 
perature of  6,°  4.9  days,  and  at  a  temperature  of  24°, 
only  a  single  day.  Stage  II  was  that  of  an  embryo 
having  a  rudimentary  medullary  plate,  with  its  edges 
rising  and  separated  by  a  broad  cleft;  Stage  III  that 
of  an  embryo  with  a  closed  medullary  tube,  and  with  a 
distinctly  marked  head:  Stage  IV  that  of  a  more  elon- 
gated embryo  with  an  obvious  tail,  but  with  gills  not 
formed;  Stage  V  that  of  an  embryo  5  mm.  long,  with 
rudiments  of  gills;  Stage  VI  that  of  an  embryo  7.5  mm. 
long,  with  well-developed  gill  tufts  and  tail  3.5  mm. 
long;  Stage  YII  that  of  an  embryo  9  mm.  long,  with  a 
tail  5  mm.  long.  The  curves  representing  the  times  of 
growth  to  all  these  more  advanced  stages  are  very  simi- 
lar to  each  other  and  to  the  first  curve. 

In  the  observations  thus  far  quoted,  the  highest  tem- 
perature employed  was  26°,  and  this  proved  to  be  also 
the  most  favourable  temperature  for  growth.  Higher 
temperatures  still  may  produce  an  adverse  influence,  as 
we  have  already  observed  in  the  case  of  Echinoid  larvae. 
For  the  growth  of  tadpoles'  tails,  Lillie  and  Knowlton 
found  the  "  optimum  "  temperature  to  be  30°,  the  rate 
of  increase  in  length  then  being  10.6  times  greater  than 
*Arch.  f.  mik.  Anat.,  li.  p.  319,  1898. 


228 


THE  EFFECT  OF  TEMPERATURE 


at  10°.  At  31°  to  34.9°,  however,  the  rate  was  only  9.0 
times  greater.  Better  instances  of  the  more  and  more 
unfavourable  influence  of  increasing  high  temperature 
are  found  amongst  plants,  as  in  them  the  optimum 
temperature  is  much  further  removed  from  the  "  maxi- 
mum "  temperature  (the  highest  temperature  at  which 
growth  can  take  place  at  all)  than  it  is  in  animals. 
The  following  table  shows  the  increments  in  the  length 
of  the  hypocotyls  of  various  plants  in  a  period  of  48 
hours,  as  determined  by  Koppen  and  by  De  Vries:* 


KOPPEN. 

DE  VRIES. 

2 

03 

1 

-B 

00 

M 
9 

ad 

'ia 

fli 

2  —  j 

a 

*3 

GO 

1! 

P-i-H 

g. 

•9J5 

§  ^ 

a 

I 

|| 

a.| 

i 

I"5 

II 

1 

m 

Jl 

fi-S 
13 

14  1°C 

9.1  mm. 

5.0  mm. 

15'.  1 

3.8  mm. 

5.9  mm. 

1.5mm. 

18.0 

11.6 

8.3 

1.1  mm. 

21.6 

24.9 

38.0 

20.5 

23.5 

31.0 

30.0 

10.8 

26.6 

54.1 

53.9 

29.6 

27.4 

52.0 

71.9 

44.8 

28.5 

50.1 

40.4 

26.5 

30.2 

43.8 

38.5 

64.6 

30.6 

44.1 

44.6 

39.9 

33.5 

14.2 

23 

69.5 

33.9 

30.2 

26.9 

28.1 

36.5 

12.6 

8.7 

20.7 

37.2 

10.0 

0.0 

9.2 

The  optimum  temperature  was  about  27°  for  every 
plant  but  one,  viz.,  Zea  mats,  and  in  this  case  it  was 
33.5°.  The  rate  of  growth  at  the  optimum  was,  in  the 
various  plants,  respectively  5.9,  10.8,  63.2,  13.7,  12.2, 
and  29.9  times  greater  than  at  the  lowest  temperature 
at  which  it  was  observed,  and  respectively  4.3,  6.2,  3.4, 

*  Quoted  from  Vines'  "  Physiology  of  Plants,"  p.  293. 


AND  OF  LIGHT. 


5.2,  and  4.9  times  greater  than  at  the  highest  tempera- 
ture. In  one  plant,  however,  the  temperature  of  37.2° 
was  sufficient  to  stop  all  growth,  so  that  this  was  a 
slightly  supra-maximal  temperature. 

That  the  increased  growth  produced  by  warmth  leads, 
at  least  in  some  cases,  to  an  actual  increase  in  the  size 
of  an  organism,  is  proved  by  some  f  urjbher  observations 
of  the  author  on  Echinoid  larvse.  'No  special  determina- 
tions were  made  as  to  the  effect  of  temperature  on 
growth,  but  it  was  noticed  that,  whilst  at  a  temperature 
of  13.8°,  the  ova  took  about  22  hours  to  reach  the  free 
swimming  blastula  stage,  they  took  only  about  5  hours 
at  24°.  We  may  assume,  then,  that  the  rate  of  growth 
increases  rapidly  with  temperature.  The  effect  of 
various  temperatures  on  the  actual  size  of  the  larvae 
may  be  gathered  from  the  following  table:* 


STRONGYLOCEN- 

SPII^ERECHINUS. 

ECHINUS  MICROTU- 

TROTUS. 

BERCULATUS. 

TEMPERATURE 

OP 
DEVELOPMENT. 

t! 

el 

<£ 

J3 

S"8) 
51 

*4 
11 

if 

11.4° 

100.0 

100.0 

100.0 

100.0 

100.0 

100.0 

15.9° 

113.5 

143.4 

109.4 

287.0 

113.4 

116.3 

20.  4* 

120.6 

156.8 

104.6 

327.2 

124.5 

106.6 

23.7° 

122.5 

149.1 

100.6 

386.7 

123.9 

113.7 

Larvae  of  three  different  species  were  allowed  to  develop 
at  four  different  temperatures,  and  measured  after  8 
days'  growth  in  respect  of  both  their  body  length  and 
their  anal  arm  length,  and  this  latter  measurement  was 
calculated  as  a  percentage  on  the  former.  In  the  case 

*Phil.  Trans.  1898,  B.  p.  479. 


230  THE  EFFECT  OF  TEMPERATURE 

of  both  Strongylocentrotus  and  Echinus  plutei,  the 
body  length  increases  considerably  with  the  tempera- 
ture up  to  20.4°,  when  it  is  more  than  20  per  cent, 
greater  than  in  plutei  grown  at  11.4°,  but  it  is  prac- 
tically unaffected  by  a  further  rise.  In  Sphcsr  echinus, 
on  the  other  hand,  the  optimum  temperature  appears  to 
be  15.9°,  and  a  further  rise  of  temperature  acts  unfav- 
ourably. The  effects  on  the  arm  lengths  differ  con- 
siderably more  than  those  on  the  body  lengths.  In 
Strongylocentrotus  this  dimension  is  half  as  long  again 
at  20.4°  as  it  is  at  11.4°,  but  in  Echinus  it  is  only  very 
little  affected.  In  the  Sph&rechinus  pluteus,  on  the 
other  hand,  it  is  nearly  four  times  longer  at  23.7°  than 
it  is  at  11.4°.  Each  organism,  therefore,  in  respect  of 
each  portion  measured,  reacts  in  a  different  manner  to 
changes  in  the  temperature  of  development. 

Some  observations  of  Standfuss  *  upon  the  larvae  of 
certain  Lepidoptera,  show  that  the  effect  of  tempera- 
ture on  growth  is  not  necessarily  in  the  direction  of  in- 
creased size.  Thus  he  found  that  whtai,  as  was  often 
the  case,  the  larval  period  was  shortened  by  raising  the 
temperature,  the  size  of  the  adult  insects  resulting 
therefrom  was  correspondingly  reduced.  For  example, 
a  pair  of  A.  fasciata,  of  which  the  wings  measured  re- 
spectively 46  and  48  mm.  across,  produced  three  speci- 
mens measuring  only  36  to  39  mm.,  when  the  larval 
stage  was  reduced  to  68  to  87  days,  and  the  pupal  to  15 
to  20  days,  by  subjection  to  a  temperature  of  25°  to 
30°.  On  the  other  hand,  some  other  eggs  from  the 
same  original  pair  of  A.  fasciata,  which,  though  exposed 

*The  Entomologist,  vol.,  xxviii.  p.  69, 1895.    (Translated  from  the 
German  by  Dr.  F.  A.  Dixey.) 


AND  OF  LIGHT.  231 

to  the  same  high  temperature,  developed  more  slowly — 
the  larval  period  taking  142  to  163  days,  and  the  pupal 
25  to  31  days — yielded  specimens  having  a  wing  meas- 
urement of  55  to  57  mm.  It  would  seem,  therefore, 
that  high  temperature  may  so  hurry  forward  the  time 
of  onset  of  the  metamorphosis  from  larva  to  pupa,  that 
there  is  insufficient  opportunity  for  adequate  larval 
feeding  and  growth,  and  a  consequent  dwarfing  of  the 
adult  imago.  If,  however,  there  be  no  curtailment  of 
the  normal  period  of  feeding,  the  high  temperature 
may  produce  a  considerable  increase  in  the  size  of  the 
individuals.  These  conclusions  are  supported  by  sev- 
eral other  observations. 

The  permanent  effects  of  temperature  on  size  are 
probably  very  considerable  among  many  of  the  Mol- 
lusca.  Thus  it  has  often  been  noticed  that  snails  living 
in  cold  and  exposed  positions  are  considerably  dwarfed 
in  comparison  with  those  living  in  warmer  regions,  but, 
as  far  as  I  am  aware,  no  exact  comparisons  have  been 
made.  Even  if  this  had  been  the  case,  it  would  not  be 
permissible  to  ascribe  the  differences  to  the  direct  result 
of  temperature,  as  this  might  have  acted  indirectly, 
through  the  vegetation.  Certain  observations  of  Mo- 
bius  *  on  marine  Mollusca  seem,  however,  to  demon- 
strate the  direct  effects  of  temperature  changes.  Thus 
it  was  noticed  that  the  Molluscs  in  the  Eastern  basin  of 
the  Baltic  are  much  more  stunted  than  those  in  the 
Western.  For  instance,  Mytilus  edulis  is  only  3  to  4 
cm.  long  near  Gothland,  whereas  at  Kiel  it  attains  a 
length  of  8  to  9  cm.  Also  in  the  Eastern  basin  the  cal- 
careous layers  of  certain  shells  such  as  M ya  arenaria 
*  Report  on  "  Pomerania  "  Expedition,  p.  138. 


232  THE  EFFECT  OF  TEMPERATURE 

are  extremely  thin.  "  These  remarkable  variations  are, 
no  doubt,  to  a  large  extent  due  to  the  violent  changes 
of  temperature  which  are  experienced  in  the  Baltic, 
and  by  which  the  steady  development  of  the  animals  in 
question  is  thrown  out  of  gear.  The  same  species  occur 
on  the  coast  of  Greenland  and  Iceland,  where  they  at- 
tain a  considerably  larger  size  than  in  the  Baltic,  in 
spite  of  the  lower  mean  temperature,  probably  because 
their  development  is  not  interrupted  by  any  sudden 
change  from  cold  to  heat,  or  vice-versa."* 

The  influence  of  a  low  temperature  on  the  colour  of 
marine  Mollusca  seems  to  be  indicated  by  the  observa- 
tions of  Fischer  f  on  the  shells  of  the  west  coast  of 
South  America.  Numerous  species  of  these  shells  ex- 
hibit a  remarkable  degree  of  melanism,  and  it  seems 
highly  probable  that  "  this  concurrence  of  specific 
melanism  (which  stands  quite  alone  in  the  world)  is  due 
to  the  cold  polar  current  which  impinges  on  the  Chilian 
coasts,  for  the  same  genera  occur  on  the  opposite  shores 
of  the  continent  without  exhibiting  any  trace  of  this 
mournful  characteristic."  $  It  is  very  improbable, 
however,  that  this  melanism  is  the  direct  result  of  the 
cold  current.  If  so,  why  should  it  not  be  observed  in 
other  parts  of  the  world,  which  are  similarly  visited  by 
cold  currents? 

More  interesting  and  unequivocal  effects  of  tempera- 
ture are  afforded  by  the  numerous  experiments  which 
have  been  made  upon  the  wing  colours  and  markings  of 

*  Quoted  from  Cooke,  "Cambridge  Natural  History,"  vol.   iii. 
p.  84. 

f  Journ.  de  Conchy!. ,  xxiii.  p.  105,  1875. 
\L.  c.,  p.  85 


AND  OF  LIGHT.  233 

Lepidoptera.  It  has  been  known  for  more  than  sixty 
years  that  the  two  butterflies  Vanessa  levana  and  V. 
prorsa,  formerly  regarded  as  different  species,  are  but 
seasonal  forms  of  one  and  the  same  species.  Thus  V. 
levana  emerges  in  the  spring,  breeds  immediately,  and 
produces  adult  V.  prorsa  progeny  in  the  same  summer. 
The  progeny  of  these  insects  pass  the  winter  as  chrysa- 
lids,  and  emerge  the  next  spring  as  V.  levana.  The 
levana  form  is  characterised  by  a  yellow  and  black  pat- 
tern on  the  upper  side  of  the  wings,  whilst  the  prorsa 
form  has  black  wings  with  a  broad  white  transverse 
band.  The  lower  surfaces  differ  only  slightly. 

It  is  a  natural  supposition  that  these  changes  of 
colour  marking  are  dependent  in  some  way  on  tempera- 
ture, and  Dorf  meister  *  proved  that  this  is  actually  the 
case.  By  the  application  of  warmth  to  the  pupae  he 
succeeded  in  producing  prorsa  out  of  the  offspring  of 
prorsa,  and  by  the  application  of  cold  he  obtained  from 
levana  not  the  pure  levana  form,  but  one  intermediate 
between  it  and  prorsa.  This  intermediate  form,  which 
has  occasionally  been  observed  in  nature,  is  termed  V. 
porima.  These  experiments  were  repeated  and  ex- 
tended by  Weismann,  and  by  employing  a  greater  de- 
gree of  cold  he  succeeded  in  obtaining  levana  from 
levana;  but  he  found  that  prorsa  was  only  exceptionally 
reared  from  prorsa  by  the  application  of  heat. 

The  mode  of  action  of  the  temperature  is  not  so  clear 
as  might  at  first  sight  be  imagined.  The  simplest  ex- 
planation is  to  attribute  the  effect  to  the  direct  influ- 
ence of  the  warmth  and  cold,  and  this  view  of  the  in- 

*Mitt.  des   naturwiss.  Vereins  fiir  Steiermark,  1864.     See  also 
Elmer's  "  Organic  Evolution,"  English  Ed.,  p.  116,  etseq. 


THE  EFFECT  OF  TEMPERATURE 


fluence  of  warmth  is  actually  held  by  Eimer.*  Accord- 
ing to  Weismann,  however,  the  action,  both  for  warmth 
and  cold,  is  an  indirect  one.  The  change  of  the  prog- 
eny of  levana  back  to  levana  through  the  influence  of 
cold  he  attributes  to  reversion  to  the  ancestral  form,  for 
there  is  practically  no  doubt  that  levana  is  phylogeneti- 
cally  the  older  form  of  the  two.  He  considers  that 
prorsa  has  slowly  arisen  through  the  gradual  increase 
in  the  warmth  of  the  climate,  or  is  a  seasonally  adaptive 
form,  and  that  its  occasional  production  from  the  prog- 
eny of  prorsa  is  due  to  the  high  temperature  unduly 
stimulating  the  development  of  the  prorsa  "  determi- 
nants." 

A  clearer  case  of  the  direct  influence  of  warmth  and 
cold  is  afforded  by  Polyommatus  phlceas,  the  Small  Cop- 
per butterfly.  By  exposure  of  the  pupse  to  various 
temperatures,  Merrifieldf  obtained  the  following  re- 
sults : 


TEMPERATURE. 

TIMB  OP 
EMERGENCE. 

COLORING  OP  SPECIMENS. 

27°—  32°  C. 

6  days 

Spots  large,   not  sharply  defined;  dusky 

suffusion  of  fore  wings. 

about  21  p 

11—15  days 

Spots  smaller;  copper  colour  more  vivid; 

black  more  intense. 

about  14Q 

22—23  days 

Copper  colour  still  more  vivid;  copper  band 

on  hind  wings  broader. 

about  7' 

57—59  days 

Effects  intensified. 

.5°;thenl3p 

10  weeks; 

Extreme  effects,  especially  in  smallness  of 

then  5  wks. 

spots  and  breadth  of  coppery  band  on 
hind  wings. 

Here  we  see  that  the  temperatures  ranged  from  about 
30°  C.,  or  85°  F.,  to  just  above  the  freezing  point.    The 


*  "  Organic  Evolution,"  p.  122. 
f  Trans.  Ent.  Soc.  1893,  p.  55. 


AND  OF  LIGHT.  235 

times  of  emergence  of  the  butterflies  from  the  chrysa- 
lis varied  from  6  days  to  no  less  than  15  weeks,  and 
probably  if  the  low  temperature  had  been  continued  in 
this  latter  case,  the  time  would  have  extended  to  many 
months,  or  there  may  have  been  no  emergence  at  all. 
It  will  be  seen  that  the  principal  effects  produced  by 
warmth  are  a  dusky  suffusion  of  the  fore  wings,  and  by 
cold  an  intensity  of  colouring  in  both  the  coppery  and 
dark  parts,  the  enlargement  of  the  copper  band  on  the 
hind  wings  being  an  especially  marked  feature.  In 
fact  these  "  cold  "  specimens  were  very  similar  to  those 
caught  in  England,  Germany,  and  similar  latitudes, 
whereas  the  "  warm  "  specimens  were  similar  to  the 
variety  eleus,  which  is  found  in  Southern  Europe. 
Merrifield  therefore  came  to  the  conclusion  that  the 
difference  in  the  appearance  of  these  local  forms  "  is 
not  necessarily  to  be  attributed  to  the  existence  of  races 
of  different  colouring,  but  may  be  owing  to  the  differ- 
ence between  the  temperatures  to  which  the  individuals 
are  exposed  in  the  two  climates."  Weismann  has 
shown,*  however,  that  the  modifications  cannot  be  en- 
tirely due  to  the  direct  effects  of  temperature.  Thus 
none  of  the  specimens  obtained  by  exposing  pupse  of  a 
German  stock  to  high  temperature  were  so  dusted  with 
black  as  the  darkest  forms  of  the  southern  variety  eleus, 
whilst  conversely,  none  of  the  specimens  obtained  by 
exposing  the  pupse  of  a  Neapolitan  stock  to  a  low  tem- 
perature were  so  light-coloured  as  the  ordinary  Ger- 
man form.  "The  German  and  Neapolitan  forms  are 
therefore  constitutionally  distinct,  the  former  tending 
much  more  strongly  towards  a  pure  reddish-gold,  and 
*  "  Germ-Plasm,"  p.  399. 


236      THE  EFFECT  OF  TEMPERATURE 

the  latter  towards  a  black  colouration."  Weismann 
thinks  that  the  two  varieties  may  have  originated  owing 
to  a  gradual  cumulative  influence  of  the  climate,  the 
slight  effects  of  one  summer  or  winter  having  been 
transmitted  and  added  to  from  generation  to  genera- 
tion. Such  a  cumulative  effect  can  be  accounted  for 
satisfactorily  by  supposing  that  the  temperature  not 
only  affects  the  "  primary  constituents  "  of  the  wings 
of  each  individual — i.  e.,  a  part  of  the  soma — but  also 
the  corresponding  "  determinants  "  of  the  germ-plasm 
contained  in  the  germ  cells  of  the  animal. 

Arguing  from  experiments  on  about  5000  pupse, 
Standfuss  *  has  endeavoured  to  classify  under  five  dif- 
ferent headings  the  effects  which  temperature  changes 
may  produce  in  Lepidoptera. 

(1)  They  may  give  rise  to  seasonal  forms  having  a 
similar  aspect  to  those  occurring  among  the  palsearctic 
fauna  at  certain  definite  seasons  of  the  year.  For  in- 
stance, pupse  of  Vanessa  c-album  (Comma  butterfly), 
kept  at  37°  C.,  gave  origin  to  the  light  coloured,  yel- 
lowish brown  form  of  butterfly,  especially  pale  on  the 
under  surface,  whilst  those  kept  in  a  refrigerator  pro- 
duced the  form  with  a  considerably  darker  under  side, 
in  many  cases  mingled  with  a  moss-green  tint.  Also 
this  form  had  much  more  sharply  defined  markings,  and 
a  more  deeply  indented  margin  to  the  wings.  Both 
these  forms,  be  it  noticed,  occur  in  nature  at  the  present 
time.  Again,  by  exposing  pupse  of  P.  machaon  (Swal- 
low-tail), to  a  temperature  of  37°  C.?  insects  were  ob- 
tained which  bore  a  perfect  resemblance  to  those  that 

*  The  Entomologist,  vol.  xxviii.  pp.  69, 102,  and  145.    (Translated 
from  the  German  by  Dr.  F.  A.  Dixey.) 


AND  OF  LIGHT.  237 

fly  in  August  in  the  neighbourhood  of  Antioch  and 
Jerusalem.  Pupae  kept  at  5°  to  8°,  however,  yielded 
the  common  Swiss  and  German  form  of  butterfly  ob- 
tained from  hibernated  pupae. 

(2)  Local  forms  and  races  such  as  occur  constantly  in 
certain  definite  localities  may  be  produced.     For  in- 
stance, exposure  of  pupae  of  V.  urticce  (Small  Tortoise- 
shell)  to  warmth  produced  specimens  somewhat  similar 
to  the  variety  ichnusa,  whilst  cold  produced  some  speci- 
mens which  strongly  recall  the  North  American  V. 
milberti,  and  others  which  were  indistinguishable  from 
the  northern  variety  polaris.    Again,  warmth  acting  on 
pupse  of  V.  cardui  (Painted  Lady),  gave  an  extraordi- 
narily pale  form,  like  those  found  in  very  different 
parts  of  the  tropics.     Cold,  on  the  other  hand,  gave 
specimens  with  a  very  recognisable  darkening  of  the 
whole  insect,  such  as  is  exhibited  by  a  form  found  in 
Lapland. 

(3)  There  may  arise  forms  of  an  entirely  similar  as- 
pect to  some  which  are  also  found  exceptionally  under 
natural  conditions,   i.    e.,   aberrations.     For   instance, 
warmth,  acting  for  a  brief  period  on  V.  cardui,  produced 
a  few  specimens  of  the  aberrant  form  elymi.     Cold, 
acting  on  pupse  of  V.  io  (Peacock),  produced  a  variety 
ab.  fischeri,  which  exhibits  a  reduction  in  the  number  of 
the  blue  scales  on  both  fore  and  hind  wings.     In  these 
and  other  characters    an  approach  to  the  type  of  V. 
urticcE  is  perceived.     Such  observations  as  these  sug- 
gest that  a  large  number  of  the  aberrations  occurring 
in  nature  may  have  actually  arisen  through  the  influ- 
ence of  abnormal  temperature  conditions. 

(4)  There    may   be   produced   phylogenetic   forms; 


238     THE  EFFECT  OF  TEMPERATURE 

forms,  that  is,  which  are  nowhere  to  be  found  on  the 
earth  at  the  present  day,  but  which  may  have  existed  at 
past  epochs.  Such  a  result  may  have  been  effected 
through  modification  of  temperature  conditions  hav- 
ing taken  place  in  the  actual  habitat  of  the  species, 
or  from  the  species  having  migrated  to  a  more  southerly 
or  northerly  region.  The  variety  fiscberi  of  V.  io,  just 
mentioned,  is  probably  a  phylogenetic  form.  The  same 
may  be  true  of  a  variety  rcederi  of  V.  antiopa  (Camber- 
well  Beauty),  which  Standfuss  obtained  by  keeping  the 
pupae  in  a  refrigerator.  Again,  exposure  of  the  pupae 
of  V.  atalanta  (Red  Admiral)  to  warmth,  produced 
specimens  approximating  towards  V.  callirrhoe  and  its 
local  forms,  such  as  var.  vulcanix,  which  are  found  in 
the  Canaries:  i.  e.,  to  forms  which  may  resemble  the 
common  ancestor  of  these  species.  Other  forms  were 
produced  which  may  perhaps  be  destined  to  arise  in  the 
future,  in  that  they  are  further  removed  from  the  type 
of  related  species,  instead  of  approximating  to  them,  like 
the  true  phylogenetic  forms.  For  instance,  the  widely 
diverging  specimens  obtained  in  a  few  instances  by  the 
action  of  warmth  on  V.  antiopa,  may  belong  to  this 
class.  This  variety  has  been  named  daubi  by  Stand- 
fuss. 

(5)  Finally,  there  is  still  a  small  unexplained  residue 
of  modifications  produced  by  temperature  changes. 
This  possibly  represents  the  direct  reaction  of  the  indi- 
vidual species,  completely  independent  of,  and  uncon- 
trolled by,  any  inherited  developmental  tendency. 

It  will  be  noticed  that  the  principle  of  reversion  is 
called  in  by  Standfuss  to  account  for  one  of  his  five 
groups,  but  Weismann,  Dixey,  Fischer,  and  others  are 


AND  OF  LIGHT.  239 

inclined  to  extend  its  scope  to  some  of  the  other  cases 
as  well.  Thus  Weismann  formerly  made  use  of  it  to 
account  for  seasonal  dimorphism,  though  now  he  rather 
withdraws  this  opinion.*  According  to  Fischer, t  both 
very  low  and  very  high  temperatures  are  equally  capa- 
ble of  determining  reversion  by  acting  simply  as  ex- 
citants. A  moderate  elevation  of  the  temperature,  on 
the  contrary,  may  give  rise  to  new  modifications  which 
are  not  phylogenetic,  but  which  actually  occur  in  warm 
climates.  Dixey,lf  arguing  especially  from  Merrifield's 
observations  on  V.  atalanta,  and  Merrifield  §  himself, 
from  these  and  other  observations,  have  come  to  the 
conclusion  that  reversion  may  be  occasioned  by  ex- 
posure both  to  high  and  to  low  temperatures,  but  that 
the  kind  of  effect  produced  is  different  in  the  two  cases. 
Eimer  is  of  the  opinion  that  only  cold  has  the  power  of 
causing  a  reversion  to  an  ancestral  form,  the  effect  of 
warmth  being  "  evidently  a  direct  effect."  ||  In  sup- 
port of  his  views,  he  refers  to  Weismann's  experiments 
on  Pieris  napi  (Green-veined  White),  and  V.  levana- 
prorsa.  The  former  butterfly  occurs  in  a  summer  and  a 
winter  form,  the  winter  being  the  darker.  There  is 
also  a  variety  of  P.  napi,  viz.,  bryonice,  which  is  found 
in  the  Swiss  Alps  and  in  the  polar  regions,  and  which 
can  be  described  as  a  very  dark  variety  of  the  winter 
form  of  P.  napi.  This  bryonicz  is  in  all  probability  the 
ancestral  form  of  P.  napi,  whilst  the  winter  form,  and 

*  The  Entomologist,  1896,  p.  240. 

f  "Transmutationen  der  Schmetterlinge  infolge  Temperaturan- 
derungen,"  Berlin,  1894. 
$  Trans.  Ent.  Soc.  1893,  p.  72. 
§  Trans.  Ent.  Soc.  1894,  p.  425. 
|  "  Organic  Evolution,"  p.  125. 


240  THE  EFFECT  TEMPERATURE 

subsequently  the  summer  form,  of  the  common  P.  napi, 
have  probably  arisen  gradually  from  it  through  the  in- 
fluence of  a  warmer  climate.  Now  Weismann  found 
that  he  was  unable  to  convert  Iryonice  into  napi  by  the 
action  of  warmth,  though  he  could  by  the  application  of 
cold  readily  change  the  summer  form  of  napi  into  the 
winter  form.  Similarly,  also,  the  progeny  of  V.  levana 
are  readily  converted  by  cold  into  levana,  but  only  ex- 
ceptionally can  the  progeny  of  V.  prorsa  be  converted 
into  prorsa.  Now,  as  already  mentioned,  levana  is 
probably  the  ancestral  form,  and  so,  in  both  this  case  and 
that  of  P.  napi,  cold  readily  produces  what  is  probably 
a  phylogenetically  older  form,  whilst  warmth  generally 
has  no  effect.  Certain  observations  by  Merrifield  * 
also  afford  some  support  to  Eimer's  view,  for  he  found 
that  "  the  capability  of  being  turned  during  the  pupal 
period  from  one  type  partially  into  the  direction  of 
the  other  exists  in  both  the  summer  and  the  winter 
type,  but  is  much  greater  in  the  former  than  in  the 
latter." 

With  regard  to  the  critical  period  at  which  tempera- 
ture especially  exerts  its  influence,  there  is  a  general 
consensus  of  opinion  that  it  is  confined  to  the  pupal 
stage,  and  in  most  cases  also  to  the  first  part  of  this 
stage.  Dorfmeister  f  concluded  that  temperature 
exerted  its  greatest  influence  during  the  change  from 
the  larval  into  the  pupal  stage,  or  shortly  afterwards. 
Weismann  J  noticed  that  in  V.  prorsa-levana  it  acted 
only  at  the  beginning  of  the  pupal  stage.  Standfuss, 

*  Trans.  Ent.  Soc.  1892,  p.  53. 

f  Vide  Eimer's  "  Organic  Evolution,"  p.  131. 

j "  Germ-Plasm,"  p.  402. 


AND  OF  LIGHT.  241 

in  almost  all  the  observations  above  referred  to,  exposed 
his  pupae  to  warmth  for  about  three  days,  and  then  kept 
them  at  the  room  temperature  until  they  emerged,  this 
generally  occurring  4  to  10  days  later.  The  exposure 
to  cold  generally  extended  to  about  30  days,  and  emer- 
gence took  place  after  about  11  days  more  at  room  tem- 
perature. As  the  effects  obtained  by  him  are  just  as 
great,  if  not  greater,  than  those  obtained  by  other  ob- 
servers, it  would  seem  quite  clear  that  in  the  forms  he 
employed  the  critical  time  for  temperature  is  cer- 
tainly during  the  first  portion  of  the  pupal  period. 
However,  Merrifield,*  in  his  observations  on  the  sum- 
mer and  winter  forms  of  P.  napi,  found  the  critical  time 
to  be  in  the  last  days  of  the  pupal  period,  a  directly 
opposite  result  to  that  of  Weismann  for  the  same  insect. 
Weismann  f  explains  the  apparent  contradiction  by 
supposing  that  in  P.  napi  adaptive  and  direct  sea- 
sonal dimorphism  are  mixed.  The  species  may  have 
adapted  itself  to  the  seasons  of  the  year  by  a  double 
protective  colouring,  and  the  critical  period  for  the  de- 
termination of  the  adaptive  form  may  be  at  the  begin- 
ning of  the  pupal  period.  The  direct  reaction  of  the 
species  to  temperature  may,  however,  as  Merrifield 
found,  be  determined  only  at  the  end  of  the  pupal 
period. 

In  his  experiments  with  P.  phl&as,  Merrifield  found 
that  pupae  kept  at  0.5°  C.  for  ten  weeks,  and  then  ex- 
posed to  a  temperature  of  32°  for  six  days,  gave  speci- 
mens with  features  very  similar  to  those  obtained  from 
pupse  kept  throughout  at  a  temperature  of  27°  to  32°. 

F  *  Trans.  Ent.  Soc.,  1893,  p.  55. 
t  The  Entomologist,  1896,  p.  240. 


242  THE  EFFECT  OF  TEMPERATURE 

The  reason  of  this  is  probably  that  a  temperature  of 
0.5°  is  so  low  that  it  paralyses  all  tissue  changes  in  the 
pupae,  and  at  the  end  of  ten  weeks  the  stage  of  develop- 
ment is  no  further  advanced  than  at  the  beginning. 
Thus  the  time  of  emergence  of  these  pupae,  after  trans- 
ference to  a  temperature  of  32°,  was  just  as  long  as  for 
those  kept  only  at  this  temperature. 

Arguing  from  his  experiments  on  two  moths,  Selenia 
illustraria  and  Ennomos  autumnaria,  Merrifield  * 
came  to  the  conclusion  that,  in  their  case  at  least,  the 
markings  were  chiefly  affected  by  the  temperature  ex- 
perienced during  the  earlier  part  of  the  pupal  period, 
whilst  the  colouring  was  "  chiefly  affected  during  the 
penultimate  pupal  stage,  i.  e.,  before  the  colouring  of 
the  imago  begins  to  show."  A  low  temperature  dur- 
ing this  latter  period  causes  darkness,  and  a  high  tem- 
perature the  opposite  effect.  Thus,  by  difference  of 
treatment,  it  was  found  possible  to  obtain  from  the  same 
brood  individuals  showing  (1)  summer  markings  with 
summer  colouring;  (2)  summer  markings  with  an  ap- 
proach toward  spring  colouring;  (3)  spring  markings 
with  summer  colouring,  and  (4)  spring  markings  with 
almost  spring  colouring. 

We  see,  then,  that  in  some  cases  seasonal  dimorphism 
is  a  direct  response  to  temperature,  or  is  a  somatic 
modification,  whilst  in  other  and  perhaps  the  majority 
of  cases  it  is  only  indirect,  the  temperature  acting  as  a 
stimulus  to  arouse  a  blastogenic  variation.  When  the 
response  is  direct,  low  temperature  generally  induces  a 
darkening  of  colour,  as,  for  instance,  in  V.  urticcZj  Las- 
ciocampa  quercus  (and  callunce),  Arctia  caja  and  E. 
*  Trans.  Ent.  Soc.  1891,  p.  55. 


AND  OF  LIGHT.  243 

autumnaria.  In  these  forms,  the  darkening  is  caused 
either  by  the  general  colour  being  obscured,  or  by  the 
size  and  general  intensity  of  the  dark  markings  being 
increased,  or  by  both  conditions.*  In  P.  phlceas,  as  we 
have  seen,  low  temperature  causes  a  lightening  of 
colour.  When  the  response  to  temperature  is  indirect, 
the  effect  is  as  often  as  not  in  one  direction  as  in  the 
other,  and  there  are  generally  more  considerable 
changes  in  the  markings,  as  well  as  in  the  general 
colouring. 

Upon  the  higher  animals  temperature  probably  acts 
but  seldom  as  a  direct  cause  of  variation.  The  white 
coat  which  many  quadrupeds  develop  on  the  approach 
of  winter  in  northern  and  arctic  climates  is  probably  in 
great  part  a  seasonally  adaptive  change,  but  it  may  also 
be  to  a  certain  extent  the  immediate,  though  perhaps 
only  indirect,  response  to  cold.  This  seems  to  be  proved 
by  an  observation  of  Sir  J.  Ross  on  a  Hudson's  Bay 
Lemming. f  This  animal  was  protected  from  the  low 
temperature  by  keeping  it  in  the  cabin,  and  had  in  con- 
sequence retained  its  summer  coat  through  the  winter. 
On  exposing  it  in  a  cage  on  deck,  where  the  temperature 
was  30°  below  zero,  the  fur  on  the  cheeks  and  a  patch 
on  each  shoulder  became  perfectly  white  during  the 
first  night.  After  another  day's  exposure  "  the  patches 
on  each  shoulder  had  extended  considerably,  and  the 
posterior  part  of  the  body  and  the  flanks  had  turned  a 
dirty  white.  .  .  At  the  end  of  a  week  it  was  entirely 
white,  with  the  exception  of  a  dark  band  across  the 
shoulders,  prolonged  posteriorly  down  the  middle  of 

*  Vide  Merrifield.     Trans.  Ent.  Soc.  1892,  p.  33. 
f  Appendix  to  Second  Voyage.    Nat.  Hist. ,  p.  xiv.  /1835.    Quoted 
from  Poulton's  "  Colours  of  Animals,"  ed.  i.  p.  94. 


244  THE  EFFECT  OF  TEMPERATURE 

the  back."  No  further  change  took  place,  and  the  ani- 
mal died  of  the  cold  a  few  days  later.  Examination  of 
the  fur  showed  that  only  the  tips  of  the  hairs  had  be- 
come white,  so  that  on  cutting  these  off,  the  coat  re- 
gained its  original  dark  colour. 

The  observations  of  F.  H.  "Welch  *  on  the  American 
Hare  (Lepus  Americanus)  throw  further  light  on  the 
nature  of  the  change.  Early  in  October  the  whiskers 
and  a  few  of  the  longer  hairs  on  the  back  were  observed 
to  become  white  at  the  tip  or  throughout.  During  No- 
vember a  new  and  rapid  growth  of  stiff  white  hairs  ap- 
peared on  the  sides  and  back,  these  hairs  being  easily 
distinguishable  from  the  autumnal  hairs  which  were 
gradually  turning  more  and  more  white,  in  that  they 
were  invariably  white  throughout.  We  have  in  this 
animal,  therefore,  a  new  white  crop  of  hairs  of  gradual 
growth,  or  a  blastogenic  variation,  stimulated  to  de- 
velop under  stress  of  cold,  and  a  rapid  and  direct  trans- 
mutation of  parts  of  the  dark  hairs  to  white;  i.  e.,  a 
somatic  modification.  Professor  Poulton  f  explains  this 
latter  change  as  an  indirect  influence  of  cold  upon  the 
nervous  system  which  presides  over  the  nutritive  and 
chemical  changes  involved  in  the  growth  of  the  hair. 
This  probably  leads  to  the  production  of  large  numbers 
of  gas  bubbles  in  the  hairs,  and  thereby  induces  an  ap- 
parent whiteness,  in  spite  of  the  fact  that  the  pigment 
is  still  present.  In  that  the  tips  of  the  hairs  are  first 
affected,  however,  rather  than  the  bases,  it  seems  to  me 
possible  that  the  cold  acts  directly  on  the  hairs  them- 
selves, and  not  indirectly  through  the  nervous  system. 

*Proc.  Zo5l.  Soc.  1869,  p.  228. 
f  "  Colours  of  Animals,"  p.  100. 


AND  OF  LIGHT. 


245 


It  should  be  pointed  out  that  some  animals,  such  as  the 
sable,  musk-sheep,  and  raven,  retain  their  dark  colour 
throughout  the  Arctic  winter,  so 
that  the  reaction  of  the  above-men- 
tioned animals  to  cold,  whether 
direct  or  indirect,  is  a  special  and 
not  a  general  phenomenon. 

Light.  The  effect  of  light  upon 
growth,  especially  in  plants,  is 
well  known  to  be  very  considera- 
ble. One  might  infer,  therefore, 
that  differences  in  the  intensity  of 
the  light  to  which  an  organism  is 
subjected  would  form  a  potent 
cause  of  variation.  Such  is  actu- 
ally the  case  among  members  of 
the  Vegetable  Kingdom,  though 
only  exceptionally  so  among  those 
of  the  Animal  Kingdom. 

If  plants  be  allowed  to  grow  in 
absolute  darkness,  they,  as  a  rule, 
become  very  much  elongated  in 
form,  whilst  their  leaves  are  small 
and  ill-shaped.  The  accompanying 
figure  shows  the  relative  growth  of 
two  seedlings  of  Sinapis  alba  of 
the  same  age,  one  of  them  reared 
in  the  dark ;  and  the  other  in  ordi- 
nary daylight.*  Sachs  found  that  potato  tubers  grown 
in  darkness  for  53  days  produced  sprouts  from  150  to 
200  mm.  high,  whilst  similar  ones  grown  in  day- 

*  From  Strasburger,  Noll,  Schenck,  and  Schimper'a  "  Textbook  of 
Botany."  Quoted  from  Davenport's  "Experimental  Morphology," 
p.  418. 


FIG.  23.— Seedlings 
of  Sinapis  alba.  E, 
reared  in  the  dark.  N, 
reared  in  ordinary 
daylight. 


246  THE  EFFECT  OF  TEMPERATURE 

light  were  only  10  to  13  mm.  high.  Again,  he  found 
that  the  hypocotyl  of  the  buckwheat  (Fagopyrum) 
reached  a  height  of  35  to  40  cm.  in  the  dark,  whilst  it 
grew  only  to  2  or  3  cm.  when  freely  exposed  to  light. 
K.  Goebel*  has  shown  that  if  cactuses  are  cultivated  in 
darkness,  their  form  changes  completely.  The  young 
shoots  are  rounded,  and  fail  to  show  the  angular  irregu- 
larities of  form  which  increase  the  surface  capable  of 
effecting  assimilation  under  the  influence  of  light. 

Darkness  conduces  to  increased  growth,  therefore,  or 
conversely,  light  tends  to  retard  growth.  That  this  is 
the  case  is  well  shown  by  an  observation  of  Wiesner.f 
This  observer  exposed  seedlings  of  the  vetch  (Vicia 
sativa)  under  a  glass  globe  to  sunlight  for  YJ  hours. 
When  placed  horizontally,  so  as  to  get  the  full  force  of 
the  sun's  rays,  no  growth  whatever  occurred,  but  when 
placed  vertically,  so  that  the  growing  part  of  the  seed- 
ling was  more  or  less  protected  by  its  leaves,  there  was 
an  increase  in  height  of  about  .8  mm.  On  the  other 
hand,  a  control  seedling  kept  in  a  darkened  globe  grew 
about  2.8  mm.  in  the  same  period.  This  retarding  effect 
of  light  is  not  universal,  however.  It  is  practically  ab- 
sent in  some  cases,  as  of  the  yam  and  of  a  wild  gourd 
(Bryonia),  and  in  those  plants  whose  rapidly  growing 
parts  are  sheltered  from  the  sun's  rays  by  protecting 
coverings  it  is  but  little  evident.  Still  Sachs'  conclu- 
sion as  to  the  effect  of  daylight  on  growth  probably  ap- 
plies with  greater  or  less  force  to  the  majority  of  plants. 
Thus  he  found  J  that  during  the  night  the  growth 

*  Flora,  Ixxxi.  p.  96. 

f  Davenport's  "Experimental  Morphology,"  p.  41. 

\  Arb.  aus  der  Bot.,  Inst.  Wiirzburg,  i.  p.  99. 


AND  OF  LIGHT.  24? 

gradually  increases,  and  reaches  a  maximum  at  day- 
break. It  then  diminishes  to  a  minimum  a  little  before 
sunset,  after  which  it  rises  again. 

It  is  not  to  be  imagined  that  because  daylight  retards 
growth  it  is  unfavourable  to  the  proper  develop- 
ment of  a  plant.  For  instance,  Karsten  *  found 
that  whilst  a  kidney  bean  reared  in  the  dark  for  a 
month  or  two  weighed  20  per  cent,  more  than  one 
reared  in  the  light,  yet  the  leaves  did  not  weigh  a 
fifth  as  much.  Again,  Clayton  t  allowed  six  bean 
plants  to  grow  in  a  spot  where  they  would  catch 
all  the  sunshine  of  the  day,  whilst  six  other  similar 
plants  were  protected  by  a  boarding,  which  effectu- 
ally screened  off  the  sun.  When  freshly  gathered 
in  October,  the  weight  of  the  beans  and  pods  of  the 
exposed  plants  was  to  that  of  the  protected  as  99  :  29, 
whilst  the  weight  of  the  dry  beans  was  as  16  :  5.  The 
next  year,  the  weight  of  the  fresh  beans  and  pods  ob- 
tained from  the  sunshine-grown  seed  of  the  previous 
year  was  half  as  much  again  as  in  the  case  of  the  plants 
from  shade-grown  seeds,  in  spite  of  the  fact  that  all  of 
the  plants  were  now  grown  in  sunshine  and  under  pre- 
cisely similar  conditions.  "  In  the  fourth  year  plants 
with  an  exclusively  shady  ancestry  produced  flowers, 
but  failed  to  mature  fruit." 

The  intensity  of  the  light  to  which  a  plant  is  exposed 
may  considerably  affect  its  form  and  structure,  as  well 
as  its  rate  of  growth.  Thus  the  effect  of  direct  sun- 
light, as  compared  with  diffused  light,  on  the  absolute 

*Landw.  Versuchs-Stat. ,  xiii.  p.  176.    Quoted  from  Davenport's 
"  Experimental  Morphology,"  p.  419. 
f  Nat.  Science,  xi.  p.  12. 


THE  EFFECT  OF  TEMPERATURE 


size  of  leaves  has  been  shown  by  Stahl*  to  consist  chiefly 
in  a  reduction  of  the  leaf  surface.  Accompanying  this 
there  is  usually  an  increase  in  the  thickness  of  the  leaf. 
In  addition  to  the  reduction  of  size,  Scott  Elliott  f  has 
shown  that  there  may  be  a  considerable  change  in  the 
form  of  the  leaves,  owing  to  the  reduction  in  the  length 
of  the  exposed  leaves  being  much  greater  than  the  re- 
duction in  the  breadth.  The  accompanying  table 
shows  the  average  ratio  of  length  to  breadth  in  from  50 
to  100  leaves  of  various  grasses  and  plants,  which  were 
collected  in  the  one  case  from  sheltered  and  shady 
places,  and  in  the  other  from  the  most  exposed  and 
driest  spots  known: 


NAME  OP  SPECIES. 

SHELTERED 
SPECIMENS. 

EXPOSED 
SPECIMENS. 

PER  CENT. 
REDUCTION. 

Stenotaphrum  glabrum, 
Paspalum  distichum,       .               .  .    J 

15.8 
23.0 

6.1 
11.0 

61.4 
52.2 

Cynadon  dactylon,          . 
Eragrostis  ciliaris,            . 

7.3 
32.4 

4.5 
17.5 

38.4 
46.0 

Cenchrus  echinatus, 

22.5 

19.8 

12.0 

Microrhyncus  sarmentosus, 

6.8 

4.7 

30.9 

Lobelia  Scaevola,     . 

2.2 

1.6 

27.3 

Psiadia  dodonsefolia,       •. 

10.2 

11.4 

+11.2 

Helichrysum  emirnese,    . 

9.4 

5.6 

40.4 

Spermacoce  globosa, 

6.0 

4.2 

30.0 

Lycium  capense,      .                        * 

1.9 

1.9 

0.0 

Brexia  madagascariensis, 

1.8 

1.5 

16.7 

Camptocarpus,  sp., 

4.1 

3.6 

12.2 

Periploca  ovata, 

1.6 

1.5 

6.2 

Commelina  nodiflora, 

3.2 

2.9 

9.4 

Tanghinia  venenifera, 

4.7 

3.6 

23.4 

Brachystephanus  cuspid  atus, 

1.7 

1.7 

0.0 

Monimia,  sp., 

2.0 

1.8 

10.0 

Sida  carpinifolia,     . 
Vinca  rosea,     ..... 

2.6 
2.3 

2.2 
2.3 

15.4 
0.0 

. 

*  Jenaisch.  Zeit.,  Bd.  xvi.  p.  102,  1882,  and  Bot.  Zeit.,  1880. 
fProc.  Linn.  Soc.,  vol.  xxviii.  (Botany),  p.  375,  1891. 


AND  OF  LIGHT.  249 

Of  these  twenty  different  species  of  grasses  and 
plants  (collected  in  Madagascar),  we  see  that  the  ex- 
posed specimens  had  a  reduced  leaf  length  ratio  in  16 
cases,  whilst  in  only  one  was  the  length  actually  in- 
creased. The  average  reduction  for  the  whole  series 
amounts  to  21.0  per  cent.,  or  is  very  considerable. 
Similar  results  to  these  have  also  been  obtained  by 
Sorauer.* 

Upon  members  of  the  Animal  Kingdom  the  direct 
effect  of  light  is  not  nearly  so  considerable.  Yung  t 
found  that  tadpoles  exposed  to  daylight  during  the  first 
25  to  60  days  of  development  were  about  16  per  cent, 
larger  than  those  kept  in  absolute  darkness.  He  found 
also  that  eggs  of  the  sea-trout,  if  reared  in  the  light, 
hatched  a  day  earlier  than  if  reared  in  the  dark,  whilst 
pond  snails  (Lymruza  stagnalis)  hatched  in  27  days  in 
the  light,  as  against  33  days  when  in  the  dark.J  It  is 
possible,  however,  that  these  effects  were  due  rather  to 
the  presence  or  absence  of  heat  rays  than  those  of 
light. 

The  most  important  influence  of  light  in  the  produc- 
tion of  variations  in  animals  lies  in  its  connection  with 
the  phenomena  of  pigmentation.  Absence  of  light 
leads  to  diminution  or  even  total  abolition  of  pigmenta- 
tion, whilst  its  presence  leads  to  an  increase  in  some 
degree  proportionate  to  the  intensity  of  the  light. 
This,  at  least,  is  the  more  or  less  direct  action  of  light. 
The  indirect  action,  through  the  intermediation  of  the 
nervous  system,  is,  as  a  rule,  exactly  the  reverse.  A 

*  Wollny's  '*  Forschungen  a.  d.  Geb.  Agricultur,"  Bd.  ix. 

f  Arch.  ZoOl.  Exper.  et  Gen.,  vii.  p.  251. 

\  Davenport's  "  Experimental  Morphology,"  p.  426. 


\ 


250      THE  EFFECT  OF  TEMPERATURE 

well-known  instance  of  the  direct  action  of  light  is 
found  in  the  bronzing  of  the  human  skin  following  on 
undue  exposure  to  the  sun;  but  to  what  extent  are  we 
entitled  to  refer  the  black  skin  of  inhabitants  of  the 
tropics  to  a  similar,  but  more  pronounced,  action? 
Eimer  *  is  of  the  opinion  that  the  effect  is  the  direct 
result  of  the  more  intense  light  and  heat.  Thus  he 
found  that  in  passing  down  the  Nile  valley  from  the 
Delta  to  the  Soudan,  the  natives  gradually  became  more 
and  more  dark-skinned,  the  further  south  they  lived. 
The  increased  light  and  warmth,  according  to  Eimer, 
lead  to  a  greater  flow  of  blood  to  the  skin,  and  the  con- 
sequent deposition  of  pigment.  This  effect  is  inher- 
ited, and  has  become  a  constant  character.  There  is, 
of  course,  no  warrant  for  laying  down  the  law  with  such 
assurance  as  this,  for  one  can  easily  imagine  several 
other  equally  possible  and  plausible  explanations  to  ac- 
count for  the  facts.  For  instance,  pigmentation  may  be 
correlated  with  a  greater  resistance  to  the  climate  of 
hot  countries,  or  with  greater  physical  strength,  and 
may  have  been  increased  by  sexual  selection.  Still, 
Eimer's  explanation  may  contain  a  distinct  modicum  of 
truth,  and  I  hope  to  prove  in  a  subsequent  chapter 
that  the  heritableness  of  acquired  characters  such 
as  increased  or  decreased  pigmentation  may  be  deduced 
without  assuming  anything  further  than  the  present 
state  of  knowledge  legitimately  warrants  us  in 
doing. 

The  diminution  or  disappearance  of  pigmentation  fol- 
lowing upon  withdrawal  of  light  is  best  illustrated  by 
reference  to  the  well-known  cave  animals.     Of  these, 
*  "  Organic  Evolution,"  p.  87. 


AND  OF  LIGHT.  251 

one  of  the  most  interesting  is  Proteus  anguineus,  which 
is  found  in  the  subterranean  caves  of  the  Karst  Moun- 
tains about  Adelsberg.  This  amphibian  is  almost 
white,  but  if  kept  for  some  time  in  the  light,  it  gradu- 
ally becomes  pigmented.  Pigment  cells  are,  in  fact, 
still  present  in  its  skin,*  and  in  all  probability  these  are 
directly  stimulated  to  exert  their  function  by  the  action 
of  the  light. 

A  similar  effect  of  exposure  to  light  has  been 
demonstrated  by  Cunningham  f  for  the  under  sur- 
face of  the  flounder  (Pleuronectes  flesus).  This  surface 
is  normally  quite  white,  but  by  keeping  young  flounders 
for  nearly  four  months  in  a  glass  dish  illuminated 
from  beneath  by  a  mirror  placed  at  a  proper  angle, 
Cunningham  found  that  10  out  of  the  13  specimens  ex- 
perimented with  developed  black  and  yellow  chromato- 
phores.  Three  of  the  specimens  showed  well-developed 
bands  of  pigment,  similar  to  those  of  the  upper  side, 
over  the  area  occupied  by  the  muscles  of  the  longi- 
tudinal fins.  Subsequently,  Cunningham  and  Mac- 
Munn  J  succeeded  in  keeping  flounders  alive  under 
these  conditions  of  illumination  for  from  9J  months  to 
nearly  two  years.  They  found  that  the  amount  of  pig- 
ment steadily  increased  with  the  duration  of  the  ex- 
posure, so  that  ultimately  almost  the  whole  of  the  lower 
side  might  become  pigmented.  This  colouration  was 
(histologically)  of  exactly  the  same  kind  as  that  of  the 
upper  side  in  normal  specimens,  though  it  was  never 
by  any  means  so  marked.  Its  production  is  more  re- 

*  Vide  Poulton's  "  Colours  of  Animals,"  p.  91. 
f  Zool.  Anzeiger,  xiv.  p.  27,  1891. 
\  Phil.  Trans.  1893,  B.  p.  765. 


252  THE  EFFECT  OF  TEMPERATURE 

markable  than  in  the  case  of  Proteus,  in  that  pigment 
cells  are  entirely  absent  from  the  skin  of  the  lower  side 
of  the  normal  Flounder.* 

The  observations  of  List  f  upon  certain  Lamelli- 
branch  Molluscs  afford  evidence  as  to  the  effects  both  of 
decrease  and  increase  of  illumination.  List  noticed 
that  various  species  of  Mytilus  (gallo-provincialis  and 
minimus),  which  had  been  collected  in  caves,  were  dis- 
tinctly less  pigmented  than  usual.  In  fact,  those  ob- 
tained from  the  extreme  ends  of  the  two  grottoes 
underneath  the  ruined  Palace  of  Donn'Anna  in  the 
Bay  of  Naples  were  all  of  them  pale  or  colourless.  In- 
dividuals of  the  same  species  were  also  found  in  the 
dark  underground  tanks  of  the  Zoological  Station,  and 
here  again  they  were  little,  if  at  all,  pigmented.  Speci- 
mens of  M.  minimus  were  even  found  in  the  pipe 
through  which  the  water  is  pumped  from  the  sea  into 
the  Aquarium,  and  these  were  characterised  by  an  ab- 
solute want  of  pigmentation.  The  converse  observa- 
tions were  made  upon  LitJiodomus  dactylus.  These 
molluscs,  which  are  usually  concealed  in  borings  in  the 
sand  of  the  sea  bottom  half  a  metre  deep,  are  pigmented 
only  at  the  tip  of  the  foot  and  the  edge  of  the  anal 
siphon,  these  being  the  only  parts  at  all  exposed  to 
light.  After  keeping  specimens  for  a  year  in  a  glass 
vessel  exposed  to  daylight,  however,  the  whole  surface 
of  the  anal  siphon  became  coloured  an  intense  red 
brown,  whilst  the  imperfect  branchial  siphon,  the  border 
of  the  mantle,  the  whole  of  the  foot,  and  the  other  ex- 
posed parts,  were  pigmented  also. 

*IUd.,  p.  767. 

f  Arch,  f .  Entwickelungsmechanik,  Bd.  viii.  p.  618,  1899. 


AND  OF  LIGHT.  253 

Again,  Vire  *  has  obtained  somewhat  similar  results 
in  his  observations  on  the  Fauna  of  subterranean  caves 
and  streamlets  in  France.  As  regards  the  Crustacea, 
he  found  that  Niphargus  virei,  which  is  of  a  rose 
colour,  after  a  few  weeks'  exposure  to  light  becomes 
covered  with  brown  spots,  and  thus  undergoes  a  rapid 
return  to  its  ancestral  form.  On  the  other  hand  Gam- 
marus  puteanus,  when  kept  for  ten  months  in  the  tanks 
of  an  underground  laboratory,  began  to  lose  its  gray- 
green  pigment,  and  after  twenty  months  most  speci- 
mens had  entirely  lost  it.  Again,  the  common  Gam- 
marus  fluviatilis,  when  kept  for  fifteen  months  under- 
ground, developed  organs  of  touch  and  smell  which  at- 
tained nearly  half  the  size  of  those  exhibited  by  the 
true  cave  Niphargus. 

The  observations  which  have  been  made  on  Amblyop- 
sis  (a  fish  found  in  the  caves  of  the  Mississippi  Valley) 
do  not  agree  with  the  above  results.  Thus  Eigen- 
mann  f  states  that  the  pigment  is  very  abundant  when 
the  young  fish  are  two  months  old,  but  even  when  these 
fish  are  kept  in  light  during  growth,  they  show  a  de- 
crease and  not  any  increase  of  pigmentation,  so  that  a 
ten-months'  fish  was  found  to  have  taken  on  the  exact 
pigmentless  condition  of  the  adult.  Both  the  pig- 
mented  condition  and  the  subsequent  depigmentation 
are  hereditarily  transmitted,  therefore,  and  seem  prac- 
tically unaffected  by  changes  of  environment  acting 
through  a  single  generation. 

*"La  Faune  souterraine  de  France,"  Paris,  1900;  vide  Abstract 
by  P.  Kropotkin  in  Nineteenth  Century,  September,  1901,  from 
which  this  reference  is  taken. 

f  Biological  Lectures,  Wood's  Holl,  1899,  p.  124, 


254  THE  EFFECT  OF  TEMPERATURE 

The  direct  dependence  of  pigmentation  on  light 
seems  to  be  proved  by  the  generality  of  the  reverse 
phenomenon  as  observed  in  cave  animals.  Whenever 
light  is  totally  excluded,  the  pigmentation  appears  to 
become  diminished  or  abolished,  whatever  class  of  the 
Animal  Kingdom  the  individuals  belong  to.  Thus 
there  have  been  found  more  or  less  unpigmented  Coe- 
lentera,  Worms,  Crustacea,  Myriapoda,  Arachnida, 
Coleoptera,  Fish,  and  other  animals  in  the  various  sub- 
terranean caves  of  Europe  and  North  America.* 
However,  the  abyssal  fauna  of  the  ocean,  occurring  at 
depths  such  that  (presumably)  no  light  can  penetrate, 
includes  numerous  species  which  are  just  as  much  pig- 
mented  as  those  exposed  to  light.  Thus  Faxon  f  di- 
vides deep  sea  Crustacea  into  two  types;  (1)  those  living 
in  the  bottom  mud,  which  are  mostly  pale  in  colour,  and 
often  blind;  (2)  those  which  swim  freely,  have  well-de- 
veloped eyes,  and  are  coloured  bright  red.  He  con- 
siders that  this  red  colour  is  due  to  the  absence  of  light 
at  these  profound  depths,  for  S.  Jourdain  $  has  shown 
that  two  different  species  of  Crustacea,  which  are  brown 
when  exposed  to  light  or  partial  darkness,  become  red 
when  placed  in  total  darkness.  MacCulloch  and  Cold- 
stream  have  suggested  a  "  theory  of  Abyssal  Light  "  to 
account  for  the  existence  at  profound  depths  of  these 
Crustacea,  and  of  the  Fish,  Mollusca,  Crabs,  and  other 
animals  with  well-developed  eyes.  This  hypothesis 
msists  essentially  in  the  idea  that  light  diffused  by 


*  Packard,  Memoirs  of  National  Academy  of  Sciences,  iv.  p.  3, 
1888. 

f  Mem.  Mus.  Harvard,  xviii.  p.  251. 
\  Comptes  Rendus,  Ixxxvii.  p.  302,  1878. 


AND  OF  LIGHT.  255 

phosphorescent  creatures  is  capable  of  taking  the  place 
of  sunlight  in  those  depths  which  the  rays  of  the  sun 
cannot  penetrate."  * 

The  more  striking  and  considerable  effects  produced 
by  light  in  members  of  the  Animal  Kingdom  are  mostly 
confined  to  cases  of  so-called  "  Variable  Protective  and 
Aggressive  Resemblance/'  or  reaction  to  the  colour  of 
the  surroundings  which  either  protects  the  animals 
from  their  enemies,  or  assists  them  to  capture  their 
prey.f  Such  a  reaction  is  rarely,  if  ever,  a  direct  re- 
sponse to  light  of  the  superficial  tissue  cells  as  a  whole, 
or  even  of  the  sensitive  pigment  cells  in  the  skin  which 
have  been  gradually  formed  in  the  course  of  evolution 
through  the  agency  of  Natural  Selection  and  other 
processes.  It  is  an  indirect  effect  produced  by  the  in- 
termediation of  the  nervous  system.  This  was  first 
proved  to  be  the  case  by  Briicke  J  for  the  chameleon, 
and  by  von  Wittich  §  for  the  frog.  The  latter  observer 
regarded  the  variations  in  colour  as  probably  reflex  in 
their  nature,  he  attributing  them  to  a  peripheral  gan- 
glionic  apparatus  in  the  skin  itself.  A  few  years  later 
Lord  Lister  |  took  up  the  problem  and  correctly  solved 
it,  he  concluding  that  in  Rana  temporaria  "  the  eyes  are 
the  only  channels  through  which  the  rays  of  light  gain 
access  to  the  nervous  system  so  as  to  induce  changes  of 
colour  in  the  skin."  The  very  conspicuous  changes 
which  can  be  produced  in  this  manner  may  be  illus- 

*  Quoted  from  Semper's  "  Animal  Life,"  p.  85. 
f  Vide  Poulton's  "  Colours  of  Animals,"  pp.  81  to  158. 
JIV.  Bd,  d.  mathemat.  naturwiss.,  Classed.  Kaiserl.  Acad.  d. 
Wissenschaft,  Wien,  1852. 
§  Muller's  Archiv.  1854,  p.  41. 
|  Phil.  Trans.  1858,  p.  627. 


256      THE  EFFECT  OF  TEMPERATURE 

trated  by  another  quotation  from  Lord  Lister's  paper: 
"  A  frog  caught  in  a  recess  in  a  black  rock  was  itself  al- 
most black;  but  after  it  had  been  kept  for  about  an  hour 
on  white  flagstones  in  the  sun,  was  found  to  be  dusky 
yellow,  with  dark  spots  here  and  there.  It  was  then 
placed  in  the  hollow  of  the  rock,  and  in  a  quarter  of  an 
hour  had  resumed  its  former  darkness.  These  effects 
are  independent  of  changes  of  temperature."  These 
changes  of  colour  have  been  shown  by  Briicke,  von  Wit- 
tich,  Lister,  and  others  to  be  due  to  the  pigment  gran- 
ules of  certain  stellate  cells  in  the  skin  varying  in  their 
degree  of  concentration  towards  the  centre  of  the  cell, 
and  in  their  diffusion  peripherally  through  the  branch- 
ing processes.  These  pigment  cells  are  often  of  differ- 
ent colours  and  are  arranged  in  layers,  so  that  widely 
different  effects  may  be  produced  by  varying  degrees  of 
concentration  in  them. 

That  the  reflex  mechanism  takes  its  origin  in  the  eye, 
which  is  stimulated  by  the  light  reflected  from  the  ani- 
mals' surroundings,  was  proved  by  Lord  Lister  in  the 
following  manner:  He  found  that  a  frog  with  its 
eyes  removed  was  totally  unaffected  by  the  colour 
of  its  surroundings.  The  nervous  system  still  re- 
tained the  capacity  for  acting  on  the  pigment  cells, 
however,  as  the  frog,  originally  dark,  became  ex- 
tremely pale  after  struggling  violently  to  escape. 
It  was  then  placed  in  a  bright  light,  but  within 
half  an  hour  became  almost  coal  black  again.  Occa- 
sionally protectively  coloured  animals  are  found  in 
nature  showing  a  total  want  of  adjustment  to  the  colour 
of  their  surroundings.  Thus  Pouchet*  noticed  that  one 
*  Quoted  by  Semper,  "  Animal  Life,"  p.  95. 


AND  OF  LIGHT.  257 

single  plaice  out  of  a  large  number  upon  a  bright  sandy 
surface  was  dark-coloured,  and  Mcoll  *  noticed  that  in 
addition  to  the  light-coloured  trout  usually  seen  in  a 
chalk  stream  in  Hampshire,  very  dark  individuals  occa- 
sionally appeared.  In  both  instances,  however,  it  was 
proved  that  the  fish  were  blind,  and  therefore  unable  to 
respond  to  the  stimulus  of  reflected  light. 

Besides  the  amphibia,  fish,  and  reptile  mentioned, 
many  other  animals  belonging  to  the  same  groups  ex- 
hibit a  similar  power  of  rapidly  adapting  their  colour  to 
that  of  their  surroundings.  The  power  is  also  pos- 
sessed by  many  invertebrate  animals.  It  is  probably 
very  common  among  Crustacea,  and  some  cuttle  fish 
can  modify  their  colours  with  extreme  rapidity.  In 
Octopus  vulgaris  the  protective  resemblance  is  very 
striking,  and  so  completely  is  it  under  the  control  of  the 
nervous  system  that  I  have  seen  an  individual  change 
its  colour  from  a  dirty  white  to  a  dark  brown  in  less 
than  a  second.  It  is  amongst  the  Lepidoptera,  how- 
ever, that  our  knowledge  has  been  furthest  advanced. 
The  power  of  adaption  has  so  far  been  proved  to 
exist  in  this  group  during  the  larval  and  pupal  stages 
only,  though  it  is  probable  that  a  relatively  small  num- 
ber of  perfect  insects  also  possess  it.f  Again,  it  is 
present  only  in  such  pupae  as  are  exposed,  and  has  been 
found  wanting  in  those  of  moths  which  are  as  a  rule 
either  buried  in  the  earth  or  concealed  in  opaque 
cocoons.  Professor  Poulton  J  has  shown,  however, 
that  the  pupa  of  the  Swallow-tailed  moth  forms  an  ex- 

*  "  Colours  of  Animals,"  p.  86. 
f  "  Colours  of  Animals,"  p.  110. 
$  Ibid.,  p.  111. 


258  THE  EFFECT  OF  TEMPERATURE 

ception  to  this  rule.  Also  he  has  found  that  the 
cocoons  themselves  may  undergo  protective  colour- 
ation. 

The  first  recorded  instance  of  variable  protective  re- 
semblance in  Lepidoptera  is  due  to  T.  W.  Wood,*  who 
in  1867  demonstrated  it  for  the  pupae  of  the  Large  and 
Small  White  butterflies  (Pieris  brassicce  and  P.  rapes). 
A  few  experiments  were  made  from  time  to  time  by 
other  observers,  but  it  was  not  until  1886  that  they 
were  undertaken  systematically  on  a  large  scale.  This 
was  done  by  Professor  Poulton,  who  obtained  most 
striking  results.f  Working  upon  over  700  chrysalides 
of  Vanessa  urticce  (Small  Tortoiseshell),  he  found  that 
pupae  placed  against  black  surroundings  became  as  a 
rule  extremely  dark,  whilst  against  white  surroundings 
"  not  only  was  the  black  colouring  matter  as  a  rule  ab- 
sent, so  that  the  pupae  were  light-coloured,  but  there 
was  often  an  immense  development  of  the  golden  spots, 
so  that  in  many  cases  the  whole  surface  of  the  pupae 
glittered  with  an  apparent  metallic  lustre."  Against  a 
gilt  background  a  much  higher  percentage  of  gilded 
chrysalides  was  obtained,  and  this  led  Professor  Poulton 
to  suggest  that  in  its  original  habitat  the  larvae  pupated 
either  against  glittering  micaceous  rocks  which  had  a 
somewhat  metallic  appearance,  or  against  dark  rough 
weathered  rocks,  and  that  they  had  acquired  the  power 
of  protectively  resembling  either  of  these  surfaces.  In 
that  such  metallic  looking  rocks  occur  over  a  compara- 
tively limited  area,  whilst  the  species  has  a  consider- 

*  Proc.  Ent.  Soc. ,  vol.  xiii. ,  3d  series,  p.  xcix.  1867. 
fPhil.  Trans.  1887,  B.  p.  811,  also  "  Colours  of  Animals,"  p.  119 
et  seq. 


AND  OF  LIGHT.  259 

able  range,  Dr.  A.  R.  Wallace  *  considers  Professor 
Poulton's  suggestion  rather  improbable;  still  it  should 
be  noted  that  the  Small  Tortoiseshell  almost  invariably 
seeks  mineral  surroundings  for  the  pupal  period,  and 
very  rarely  becomes  a  chrysalis  on  its  food  plant. 

The  time  at  which  the  colours  are  determined  was 
found  to  be  especially  during  the  resting  stage  of  the 
caterpillar,  just  before  pupation,  and  to  a  less  degree 
during  the  onset  of  the  pupal  stage,  when  the  caterpillar 
hangs  head  downwards,  suspended  by  its  last  pair  of 
claspers.  The  former  stage  lasts  about  15  hours,  and 
the  latter  about  18,  and  at  the  end  of  it  the  skin  splits 
along  the  back  of  the  head,  and  the  chrysalis  becomes 
exposed.  The  reaction  of  the  skin  of  the  larva  to  the 
colour  of  its  surroundings  was  proved  by  some  ingenious 
experiments  to  be  an  indirect  one,  effected  probably 
through  the  medium  of  the  nervous  system.  Thus 
when  a  larva,  during  the  onset  of  the  pupal  stage,  was 
so  placed  that  part  of  it  was  illuminated  by  a  gilded 
background,  and  part  by  a  black  one,  parti-coloured 
pupae  were  never  obtained.  The  effective  results  were 
produced  by  that  colour  to  which  the  larger  area  of  skin 
had  been  exposed. 

We  see,  then,  that  in  the  development  of  certain  Lepi- 
doptera  there  is  a  period,  lasting  only  a  day  or  two,  dur- 
ing which  an  extreme  sensitiveness  to  the  colour  of  the 
surroundings  is  present,  and  we  have  also  seen  that  dur- 
ing the  pupal  period  there  may  be  a  great  sensitiveness 
to  the  temperature  of  the  surroundings.  These  cases 
therefore  form  exceptions  to  the  conclusion  arrived  at 
in  the  last  chapter,  viz.,  that  reaction  to  environment 
*  "  Darwinism,"  p.  198. 


260  THE  EFFECT  OF  TEMPERATURE 

diminishes  regularly  with  progress  of  development.  It 
is  obvious,  however,  that  both  these  capacities  for  re- 
action are  quite  unusual,  and  have  been  specially  ac- 
quired for  a  special  purpose.  In  all  probability  the 
organisms  are  not  more  sensitive  to  environmental  con- 
ditions in  general  at  these  periods  than  they  are  at  the 
earlier  ones;  in  fact,  they  are  probably  very  much  less 
so,  in  that  the  growth  has  almost  ceased. 

In  certain  caterpillars  the  existence  of  a  variable  pro- 
tective resemblance  has  long  been  recognised,  several 
instances  of  the  phenomenon  being  collected  by  Mel- 
dola  *  in  1873.  For  example,  the  larvae  of  the  genera 
SmerintTius  and  Sphinx,  which  are  green  when  feeding 
on  their  respective  food  plants,  become  brown  previous 
to  pupation,  when  the  caterpillars  are  crawling  over  the 
ground  to  find  a  suitable  burying  place.  Again,  the 
Geometer  Acidalia  degeneraria  is  greenish  brown  in  the 
summer,  but  changes  to  a  rusty  brown  in  the  autumn, 
at  the  period  preparatory  to  hibernation.  Some  years 
later,  Meldola  recorded  an  observation  by  Mr.  E. 
Boscher,f  relative  to  the  larvae  of  Smerinthus  ocellatus 
(Eyed  Hawk  Moth).  These  larvae  were  noticed  to  be  of 
a  whitish  green  colour  when  feeding  on  one  species  of 
willow,  and  of  a  bright  yellowish  green  when  feeding 
on  another  species,  these  colours  being,  on  the  whole, 
protective.  It  was  generally  believed  that  such  .varia- 
bility in  the  colour  of  caterpillars  is  due  to  the  direct 
chemical  effect  of  the  different  kinds  of  leaves  eaten, 
but  Professor  Poulton,J  by  his  experiments  on  the 

*Proc.  Zool.  Soc.  1873,  p.  153. 

f  Weismann's  "  Studies  in  the  Theory  of  Descent,"  1882,  p.  241. 

i"  Colours  of  Animals,"  p.  149. 


AND  OF  LIGHT.  261 

larvae  of  Smerinthus,  proved  that  it  was  the  colour  of 
the  leaves,  and  not  their  food  quality,  which  provoked 
the  change.  Thus  he  sewed  leaves  together,  "  so  that 
the  caterpillars  were  exposed  to  the  colour  of  the  upper 
or  of  the  under  side  alone,  although  they  ate  the  same 
leaf  in  both  cases.  In  other  instances  the  bloom  was 
rubbed  off  the  under  sides  of  some  leaves,  whilst  others 
were  left  normal." 

More  striking  cases  of  protective  resemblance  were 
obtained  by  Professor  Poulton  for  various  Geometra 
larvae.  Larvae  surrounded  by  the  leaves  on  which  they 
fed,  became,  in  the  majority  of  species,  light  brown  or 
light  gray  in  colour.  If,  however,  an  abundance  of 
twigs  had  been  mixed  with  the  leaves  of  the  food  plant, 
they  became  dark  in  colour.  The  larvae  of  the  Pep- 
pered moth  (Amphidasys  betularia)  afforded  the  most 
striking  result  of  all,  for  when  reared  amongst  green 
leaves  and  shoots  they  became  bright  green  without  ex- 
ception, whilst  in  the  presence  of  dark  brown  twigs  they 
nearly  all  assumed  a  corresponding  colour. 

The  influence  of  the  surroundings  acts  only  very 
slowly  upon  the  colour  of  the  caterpillars,  the  coloured 
part  being  "  actually  built  up  of  the  appropriate  tint." 
Probably  this  is  the  result  of  light  stimuli  acting  on  the 
surface  of  the  skin,  and  not  reflexly  through  the  eye. 
Thus  painting  the  eyes  (ocelli)  with  opaque  varnish  led 
to  no  diminution  of  reaction. 


CHAPTER   VIII. 

THE   EFFECT   OF   MOISTURE    AND    OF    SALINITY. 

Effect  of  humidity  of  soil  on  plant  growth — Effect  of  dry  and  moist 
surroundings  on  characters  of  plants — Desert  plants  and  Aquatic 
plants — Effect  of  moisture  on  Lepidoptera  and  on  Molluscs — 
Characters  of  maritime  plants  probably  due  to  saline  environment — 
Conversion  of  A.  saUna  into  A.  milhausenii  and  into  BrancMpus — 
Effect  of  increased  salinity  on  characters  of  the  cockle— Influence 
of  salinity  on  rate  of  growth  of  Tubularians,  and  on  size  of  sea- 
urchin  larvae. 

IN  that  the  presence  of  water  is  absolutely  necessary 
to  enable  living  organisms  to  exhibit  activity,  and  very 
probably,  indeed,  to  enable  them  to  retain  vitality  at  all 
— for  even  spores  contain  a  small  percentage  of  water — 
so  we  should  conclude  that  differences  in  the  amount  of 
water  in  the  environment  of  the  organisms  would  form 
a  fertile  source  of  variation.  Such  is,  in  fact,  the  case 
in  the  Vegetable  Kingdom.  For  example,  the  amount 
of  water  in  the  soil  has  a  considerable  influence  on  the 
rate  of  growth,  as  is  shown  in  the  table  given  below. 
These  figures,  which  were  obtained  by  Hellriegel,* 
represent  the  amount  of  dry  substance  contained  in  the 
grain  and  chaff  of  barley  which  had  been  reared  in  soils 
containing  various  percentages  of  the  saturation  quan- 
tity of  moisture.  We  see  that  the  rate  of  growth 
varies  but  little  until  the  humidity  falls  below  30  per 
cent.,  and  then  it  diminishes  so  rapidly  that  with  a  hu- 
*  Quoted  from  Davenport's  "  Experimental  Morphology,"  p.  353. 


THE  EFFECT  OF  MOISTURE  263 

midity  of  10  per  cent,  it  has  almost  ceased.  In  the 
observations  made  by  Gain,*  the  fresh  weight  of  the  en- 
tire plant  was  determined.  Seeds  of  various  species 
were  planted  in  soil  containing  either  from  3  to  6  per 
cent,  of  water,  or  from  12  to  16  per  cent.  Growth  was 
more  rapid  in  the  damp  than  in  the  dry  soil,  so  that  the 
weight  of  the  full-grown  plant  was  1.12  times  greater 
in  the  radish,  2.23  times  in  the  bean,  and  2.7  times  in 
the  flax. 


PRODUCTION  IN  DRY  SUBSTANCE. 

HUMIDITY  OF  BOIL. 

GRAIN. 

CHAFF. 

per  cent. 

80 

8.8 

9.5 

60 

10.0 

11.0 

40 

10.5 

9.6 

30 

9.7 

8.2 

20 

7.7 

5.5 

10 

.7 

1.8 

5 

.0 

.1 

The  effect  of  a  dry  soil  and  atmosphere  is  well  shown 
by  the  characters  of  desert  plants.  These  are  stunted 
in  growth,  and  are  of  a  nearly  uniform  gray  colour,  ow- 
ing to  their  intense  hairiness.  The  leaves  are  more 
fleshy,  and  there  is  a  great  tendency  to  the  formation 
of  spines.  That  these  characters  are  in  part  at  least 
the  direct  result  of  want  of  water  is  shown  by  the 
fact  that  they  may  disappear  if  an  abundance  of  water 
is  supplied.  Thus  Ononis  spinosa.  L.,  if  grown  in  a 
rich,  well-watered  soil,  or  in  a  moist  atmosphere,  gradu- 
ally loses  its  spines,  those  first  formed  under  the  new 
*  Ann.  Sci.  Nat.  Bot.  (7),  xx.  p.  63,  1895. 


264  THE  EFFECT  OF  MOISTURE 

conditions  being  much  reduced  in  size  and  rigidity.* 
Lothelier  f  has  made  numerous  observations  in  which 
individuals  of  the  same  species  were  placed  side  by  side, 
some  exposed  freely  to  the  air,  and  others  kept  moist 
under  a  glass  shade  by  a  vessel  of  water.  He  found 
that,  for  instance,  Berberis  vulgaris  bore  non-spinescent 
leaves  in  a  moist  atmosphere,  but  spines  and  spines 
alone  in  a  perfectly  dry  one.  Again,  the  shoots  which 
in  Lycium  barbarum,  Ulex  europceus,  etc.,  would  nor- 
mally have  formed  thorns  by  arrest  in  development  and 
sclerosis,  in  a  very  damp  atmosphere  continued  to  grow, 
and  elongated  into  leafy  branches.  Microscopical  ex- 
amination showed  that  in  the  moist  atmosphere  the 
parenchyma  was  only  imperfectly  differentiated  into 
spongiform  and  palisade  tissue,  whilst  in  dry  air  there 
was  a  great  arrest  in  the  area  of  parenchymatous  tissue, 
but  the  palisade  cells  were  well  developed,  and  there 
was  a  special  consolidation  of  fibrous  tissues.  Again, 
the  common  water-reed,  Phragmites  communis,  when 
growing  in  the  unirrigated  areas  of  the  Mle  Valley, 
forms  a  stunted  growth,  with  very  short  and  sharp- 
pointed  leaves.  "  Close  to  the  Mle,  however,  in  Ehoda 
Island,  it  grows  nine  or  ten  feet  high,  with  long  leaves 
almost  exactly  like  the  plants  in  English  rivers."  $ 
The  effect  of  drought  upon  Dioscorea  batatus  (Yam) 
has  been  carefully  studied  by  Duchartre.§  Though 
not  allowed  to  have  any  water,  some  tubers  of  this  plant 

*Rev.  G.  Henslow,  "  The  Origin  of  Plant  Structures,"  p.  40. 
f  See  also  "  Recherches  anatomiques  sur  les  epines  et  les  aiguillons 
des  plantes,"  Lille,  1893. 

\  "  Origin  of  Plant  Structures,"  p.  41. 

§  Bull,  de  la  Soc.  Bot.  de  France,  1885,  p.  156. 


AND  OF  SALINITY.  265 

produced  long  shoots.  The  stem  was  more  slender  than 
usual,  but  excessively  rigid,  owing  to  the  reduction  of 
the  parenchymatous  tissues,  and  the  predominance  of 
the  elements  of  consolidation.  The  leaves  were  small 
and  undifferentiated,  and  the  stomata  undeveloped. 

Many  other  instances  showing  the  relations  between 
floral  structures  and  arid  surroundings  have  been  col- 
lected by  Henslow  in  his  book  on  the  "  Origin  of  Plant 
Structures,"  where  the  subject  is  dealt  with  in  extenso. 

The  effects  of  a  very  great  increase  in  the  humidity 
of  the  surroundings,  such  as  is  experienced  by  plants 
which  actually  live  in  water,  lie,  as  might  be  expected, 
in  a  very  different  direction.  That  the  peculiar  char- 
acters of  aquatic  plants  are  in  considerable  measure  the 
direct  effects  of  their  peculiar  environment,  is  proved 
by  the  fact  that  plants,  normally  terrestrial,  often  de- 
velop such  characters  when  grown  in  water.  Thus 
Costantin  found  that  under  such  conditions  a  diminu- 
tion in  the  number  of  the  vessels  of  the  fibro-vascular 
system  of  the  stem  invariably  occurred.  For  instance, 
in  Vicia  sativa  (Vetch)  the  middle  of  the  stem  of  the 
aquatic  form  of  the  plant  had  only  38  vessels,  whilst  the 
aerial  form  had  47.  In  Ricinus  communis  (Castor-oil 
plant)  the  aquatic  form  had  10,  as  against  21;  in  Faba 
vulgaris  it  had  2  at  the  sides  and  15  at  the  angles,  as 
against  respectively  5  and  36.  The  pseudo-aquatic 
forms  thus  tended  towards  true  aquatic  plants,  in  which 
the  fibro-vascular  system  is  always  more  or  less  de- 
generate. Again,  Costantin  found  that  there  was  an 
increase  in  the  lacunae,  when  stems  normally  aerial  are 
kept  submerged,  just  as  the  aquatic  form  of  amphibious 
plants  is  found  to  have  more  of  such  lacunae  than  the 


266  THE  EFFECT  OF  MOISTURE 

aerial.  As  regards  the  leaves,  it  is  well  known  that 
when  aerial  and  floating  leaves  are  present  on  the  same 
aquatic  plant,  they  differ  greatly  in  structure,  and  as  a 
rule  also  in  form,  from  the  submerged  leaves.  In 
Ranunculus  heteropJiyllus  and  Cabomba  aquatica,  for 
instance,  the  floating  leaves  are  more  or  less  rounded, 
whilst  the  submerged  ones  have  dissected  and  filiform 
segments.  In  Hippuris  (Mare's  tail)  the  aerial  and 
floating  leaves  are  short,  and  in  CallitricJie  rounded, 
but  the  submerged  leaves  of  both  are  linear  or  ribbon- 
like.  In  all  cases  the  submerged  leaves  are  of  a 
more  delicate  texture,  more  or  less  translucent,  and  of 
a  brighter  green  colour  than  the  others.  They  show 
degradation  of  anatomical  structure  in  every  part,  the 
cuticle  and  stomata  disappearing,  whilst  the  chlorophyll 
grains  and  the  mesophyll  are  greatly  reduced  in  quan- 
tity.* Even  better  evidence  of  the  direct  relation  be- 
tween environment  and  character  is  afforded  by  certain 
other  observations  of  Costantin.  Thus  he  found  that 
he  could  change  the  form  of  Hippuris  at  will,  "  by 
transplanting  an  aquatic  plant  on  to  land,  and  vice 
versa;  all  the  leaves  produced  under  water  were  long, 
undulated,  and  delicate;  whereas  those  in  air  were  short, 
erect,  and  firm."  Again,  he  found  that  the  leaves  of 
Sagittaria  (Arrowhead),  when  deeply  submerged,  are 
soft  and  flexible,  and  may  reach  a  length  of  over  six 
feet,  but  when  developed  in  air  they  are  short  and 
erect. 

When  a  leaf  is  full  grown,  sudden  change  of  environ- 
ment kills  it,  aerial  leaves  perishing  under  water,  and 

*F^Henslow's  "Origin  of  Plant  Structures,"  chap,  viii.,  from 
which  the  greater  part  of  this  paragraph  is  drawn. 


AND  OF  SALINITY.  267 

aquatic  ones  perishing  in  air;  but  if  it  is  only  in  the 
course  of  its  development,  it  can  adapt  itself  to  a 
changed  environment.  Thus,  if  a  half -formed  floating 
leaf  of  Ranunculus  heterophyllus  or  of  Sagittaria  is 
submerged,  "it  is  at  once  arrested,  and  begins  to  re- 
adapt  itself  to  water."  There  are  some  species,  how- 
ever, such  as  many  algae,  which  show  no  power  of  adap- 
tation, and  can  only  live  entirely  under  water. 

The  converse  experiments  of  growing  aquatic  plants 
on  land  afforded  equally  striking  results.  For  instance, 
it  was  pointed  out  by  Godron,*  as  long  ago  as  1839,  that 
whilst  Ranunculus  aquatilis  (Water  crowfoot),  when 
wholly  submerged,  has  all  its  leaves  delicately  lacini- 
ated,  yet  "  if  the  plant  is  able  to  send  some  of  its  leaves 
to  the  surface,  they  float  and  assume  a  very  different 
form,  being  kidney-shaped  and  lobed.  The  same  plant 
when  growing  entirely  out  of  water  presents  a  very  dif- 
ferent appearance;  the  stem  is  short,  much  divided  into 
branches,  which  bear  a  large  number  of  small  leaves, 
cylindrical,  much  divided,  and  somewhat  thick.  If  it 
were  not  for  the  floral  organs,  one  would  certainly  be- 
lieve in  two  or  three  species."  Again,  Costantin  grew 
a  plant  of  the  aquatic  form  of  Peplis  Portula  on  land, 
and  found  that  the  internodes  were  changed  from  their 
elongated  form  to  a  short  one.  The  septa  of  the  cor- 
tical parenchyma  of  the  stem  remained  homogeneous, 
instead  of  being  hollowed  out  into  secondary  lacunae, 
and  also  the  number  of  vessels  was  increased.  Thus 
there  were  53  vessels  in  the  land  form  of  Peplis  Por- 
tula, instead  of  25;  12  instead  of  4  in  Callitriche,  and 
57  instead  of  18  in  Nasturtium. 
*  Quoted  from  De  Varigny's  "Experimental  Evolution,"  p.  97. 


268  THE  EFFECT  OF  MOISTURE 

It  may  have  been  noticed  that  in  speaking  of  these 
adaptations  of  terrestrial  plants  to  water,  and  of 
aquatic  plants  to  land,  it  has  been  more  or  less  tacitly 
assumed  that  the  effects  observed  were  due  to  the  direct 
influence  of  the  surroundings  on  the  tissues.  It  is  of 
course  possible  that  they  are  partly  or  even  largely  in- 
direct, and  that  the  change  of  habitat  merely  calls  up 
latent  characters  long  since  possessed  by  the  ancestral 
plants  which  lived  in  similar  surroundings. 

Upon  members  of  the  Animal  Kingdom,  observa- 
tions as  to  the  effect  of  moisture  are  exceedingly 
meagre.  This  is  probably  attributable  to  the  fact  that 
in  most  cases  a  direct  effect  is  either  slight  or  wanting. 
Thus  Merrifield  *  could  not  observe  any  influence  upon 
the  pupae  of  certain  Lepidoptera  (E.  autumnaria  and 
8.  illustraria),  nor  could  Standfuss  upon  those  of  cer- 
tain other  species.  Koch,f  however,  came  to  the  con- 
clusion that  a  long  period  of  dry  or  moist  weather  might 
exercise  a  considerable  influence  on  the  size  of  the  suc- 
ceeding generation.  Immediately  after  a  continuously 
dry  summer,  butterflies  are  always  smaller  than  after 
a  moist  one.  Likewise  also  the  second  generation  of 
Argynnis  selene,  which  takes  flight  in  the  height  of  sum- 
mer, is  always  smaller  than  the  spring  generation;  but 
it  seems  to  me  highly  probable  that  these  effects  are  of 
an  indirect  nature,  dependent,  perhaps,  on  changes 
effected  by  the  moisture  in  the  vegetation  on  which  the 
larvae  feed. 

Leydig  $  has  endeavoured  to  trace  a  connection  be- 

*  Trans.  Ent.  Soc.  1891,  p.  163. 

f  Quoted  fromEimer's  "  Organic  Evolution,"  p.  152. 

i  "  Organic  Evolution,"  p.  97. 


AND  OF  SALINITY.  269 

tween  the  moisture  in  the  environment  and  the  dark- 
ness of  colouring  of  certain  animals.  Thus  he  observed 
that  Molluscs  such  as  Arion  empiricorum  (common 
slug),  Helix  arbustorum,  Succinea  Pfeifferi,  and  Helix 
circinata  became  darker  than  usual  in  moist  localities. 
He  observed  a  similar  condition  also  in  certain  Am- 
phibia and  in  Lacerla  vivipara.  However,  Eimer  ob- 
served just  the  reverse  condition  in  Arion,  finding 
it  darker  upon  the  heights,  where  there  was  little  water, 
than  in  well-watered  valleys.  In  any  case,  the  effect  is 
probably  an  indirect  one,  acting  through  the  vegetation. 
The  Effect  of  Salinity.  The  effect  of  salinity  upon 
members  of  the  Vegetable  Kingdom  is  well  illustrated 
by  the  peculiarities  of  form  and  structure  possessed  by 
maritime  plants.  That  these  characters  are  at  least  in 
part  the  direct  effect  of  the  salinity  of  the  soil  and  at- 
mosphere, is  proved  by  comparison  of  plants  growing 
near  the  sea-shore  with  individuals  of  the  same  species 
growing  inland.  Thus  Lesage  *  has  investigated  no 
less  than  85  different  species.  He  found  that  in  54  of 
them  the  leaves  were  thicker  in  the  maritime  indi- 
viduals than  in  the  inland  ones,  they  being  about  four 
times  as  thick  in  Cakile  maritima  and  Silene  mari- 
tima;  in  27  there  was  no  apparent  difference,  and  in  4 
they  were  thicker  in  the  inland  individuals.  With  re- 
gard to  the  mesophyll,  there  was  no  noticeable  change 
in  11  species,  but  in  all  the  other  shore  plants  the  pali- 
sade cells  were  more  numerous  or  attained  greater 
thickness,  and  at  the  same  time  the  interspaces  under- 
lying the  stratum  of  palisade  cells  were  much  reduced. 

*"  Influence  du  Bord  de  la  Mer  sur  la  Structure  des  Feuilles," 
Rennes,  Oberthur,  1890.    Also  Rev.  Gen.  de  Bot.,  torn.  ii.  p.  54. 


270  THE  EFFECT  OF  MOISTURE 

Changes  in  the  epidermis  were  much  less  frequent, 
there  being  no  appreciable  difference  in  31  plants.  In 
23  of  the  shore  plants  the  cells  were  larger,  however, 
they  being  three  to  five  times  as  large  in  Beta  vulgaris 
and  Silene  maritima.  In  four  instances  these  cells 
were  larger  in  the  inland  plants.  With  regard  to  the 
chlorophyll,  there  was  no  difference  in  some  cases,  but 
in  others  it  was  marked.  Thus  in  Thesium  humifusum 
and  CaJcile  maritima  the  grains  were  much  smaller  in 
the  maritime  plants,  and  in  other  species  the  number  of 
grains  was  reduced. 

Even  more  conclusive  evidence  of  the  direct  effect  of 
salinity  in  producing  these  peculiarities  of  structure 
has  been  afforded  by  experiment.  Lesage  cultivated 
various  plants  under  similar  conditions  except  that 
some  of  them  were  watered  with  water  containing  com- 
mon salt,  and  he  found  that  characters  were  developed 
similar  to  those  exhibited  by  maritime  plants.  In 
Pisum  sativum  the  leaves  increased  in  thickness,  pali- 
sade cells  became  larger  and  more  numerous,  whilst  the 
intercellular  spaces  and  the  chlorophyll  diminished. 
Lepidium  sativum  (Garden  Cress)  gave  even  more 
marked  results.  The  palisade  tissue  was  more  devel- 
oped and  possessed  an  extra  layer;  the  lacunae  were  less 
pronounced,  and  the  chlorophyll  less  abundant.  On 
sowing  the  seeds  of  this  plant  a  second  year,  moreover, 
and  again  treating  the  plants  with  salt  water,  a  still 
more  marked  result  was  obtained,  it  appearing  as  if  the 
alteration  in  the  tissues  of  the  second  generation  was 
carried  on  more  or  less  from  the  point  gained  in  the 
first.  The  salted  water  might  even  affect  physiological 
processes.  Thus  radishes  usually  contain  no  starch, 


AND  OF  SALINITY.  271 

but,  after  treatment  with  salted  water  (.3  to  1  per  cent, 
in  strength),  might  contain  a  great  deal.  On  the  other 
hand,  watering  cress  with  1  per  cent,  solution  caused 
the  starch  normally  present  to  disappear,  either  wholly 
or  in  part. 

Upon  the  growth  and  even  on  the  actual  structure  of 
animals,  changes  of  salinity  may  in  some  instances  exert 
a  marked  action.  Animals  accustomed  to  develop  in 
fresh  water  have  their  growth  retarded  by  the  addition 
of  salt.  Thus  Yung  *  reared  frog's  embryos  in  solu- 
tions containing  respectively  0,  .2,  .4,  .6,  and  .8  per 
cent,  of  salt,  and  found  that  except  in  the  .2  per  cent, 
solution,  which  had  no  influence,  there  was  a  retarda- 
tion in  development.  This  increased  with  the  concen- 
tration of  the  solution,  so  that  in  the  .8  per  cent,  solu- 
tion the  larvae  took  17  days  longer  to  hatch  than  in 
pure  water.  Again,  Sargeant  f  found  that  the  rate  of 
reproduction  by  fission  of  the  naid  Dero  vaga  becomes 
slower  and  slower  according  to  the  concentration  of  the 
solution  it  is  reared  in.  Taking  the  rate  in  pure  water 
as  11.3,  it  becomes  reduced  to  8.5  in  .05  per  cent,  solu- 
tion of  salt,  to  7.7  in  .1  per  cent,  solution,  4.1  in  .2  per 
cent  solution,  and  .3  in  .3  per  cent,  solution.  Still 
stronger  solutions  stop  reproduction  altogether,  and  kill 
off  some  of  the  worms. 

The  more  considerable  effects  which  change  of  sa- 
linity may  produce  are  well  illustrated  by  the  interest- 
ing and  widely  known  observations  of  Schmanke- 
witsch  J  upon  Artemia  salina  and  A.  milkausenii. 

*Arch.  des  Sci.  Phys.  et  Nat.,  xiv.  p.  502,  1885. 

f  Davenport's  "  Experimental  Morphology,"  p.  365. 

JZeit.  f.  wiss.  Zool.,  xxv.  p.  103,  1875,  andxxix.  p.  429,  1877. 


272  THE  EFFECT  OF  MOISTURE 

The  history  of  these  observations  is  as  follows: 
Through  the  breaking  of  a  dam  across  a  salt  lake 
(Kuyalink),  a  number  of  individuals  of  A.  salina  were 
washed  from  the  upper  less  saline  waters  into  the  lower 
more  concentrated  waters,  and  the  Specific  Gravity  of 
these  was  at  the  same  time  reduced  to  1.058.  After 
the  dam  was  repaired  the  concentration  gradually  in- 
creased again  through  evaporation,  the  Specific  Gravity 
rising  to  1.105  the  year  after;  to  1.135  the  next  year, 
and  to  1.205  the  year  after  that.  Accompanying  this 
concentration,  the  generations  of  Artemia  progressively 
degenerated,  till  they  finally  attained  the  characters  of 
A.  milhausenii.  Schmankewitsch  also  succeeded  in  con- 
verting a  brood  of  A.  salina  into  A.  milhausenii  by  the 
artificial  process  of  gradually  increasing  the  percentage 
of  salt  in  the  water  in  which  they  were  living  (the 
Specific  Gravity  being  raised  from  1.028  to  1.205). 

These  two  forms  of  Artemia  have  been  held  to  be 
distinct  species,  in  that  milhausenii  shows  an  absence  of 
fins  and  bristles  on  the  lobes  of  the  tail,  and  has  much 
smaller  tail  lobes,  but  larger  branchial  appendages  to 
the  legs,  than  salina.  Schmankewitsch  himself  did  not 
hold  this  opinion,  however,  and  Bateson,*  who  has 
recently  studied  the  question  afresh,  thinks  similarly. 
Bateson  collected  samples  of  Artemia  from  a  number  of 
different  salt  lakes  in  Western  Central  Asia  and  West- 
ern Siberia,  and,  consonant  with  Schmankewitseh's 
statement,  he  found  that,  on  the  whole,  the  number  of 
bristles  on  the  caudal  fins,  and  likewise  the  size  of  the 
{ins,  was  smallest  in  specimens  collected  from  waters  of 
high  Specific  Gravity.  The  accompanying  table  shows 
*"  Materials  for  the  Study  of  Variation,"  p.  96. 


AND  OF  SALINITY.  273 

the  range  in  the  number  of  bristles  on  a  single  fin,  only 
adult  females  bearing  eggs  in  the  ovisac  having  been 
reckoned.  Only  the  Specific  Gravities  of  the  various 
waters  are  given,  but  their  chemical  composition  varied 
between  even  wider  limits,  and  this  may  be  responsible 
for  some  of  the  irregularities  observed: 


SI'.  0. 

BRISTLES. 

SP.  0. 

BRISTLES. 

SP.   0. 

BRISTLBS 

1.030 

10—24 

1.100 

4—10 

1.160 

16—19 

1.050 

11—13 

1.105 

5—9 

1.165 

1—3 

1.056 

9—17 

1.105 

4-8 

1.165 

1—5 

1.065 

2—7 

1.115 

1—6 

1.170 

6—8 

1.075 

8—13 

1.115 

5—9 

1.175 

1-5 

1.075 

5—7 

1.130 

12—16 

1.179 

4—9 

1.085 

13—15 

1.140 

3—7 

1.204 

2—5 

1.095 

20—28 

1.150 

4—10 

1.215 

2-4 

1.100 

8—14 

1.150 

7-8 

1.215 

2—7 

1.100 

8—12 

1.150 

0—1 

As  this  table  and  also  Schmankewitsch's  results  show, 
there  is  no  true  differentiation  between  A.  salina  and 
A.  milhausenii,  in  that  these  extreme  forms  are  con- 
nected by  a  continuous  series  of  naturally  occurring 
intermediate  forms.  As  Bateson  remarks,  "  it  has 
never  been  shown  that  there  is  a  male  A.  milhausenii, 
with  distinctive  sexual  characters,  and  among  the 
Branchiopoda  the  various  sexual  characters  of  the 
second  antennae  in  the  male  are  most  strikingly  dis- 
tinctive of  the  several  forms." 

In  addition  to  the  particular  character  in  question, 
Bateson  found  that  there  was  a  great  variation  in  other 
characters  as  well.  Thus  he  says :  "  Almost  each 
locality  has  its  own  pattern  of  Artemia,  which  differs 
from  those  of  other  localities  in  shades  of  colour,  in 
average  size,  or  in  robustness,  and  in  the  average  num- 


274  THE  EFFECT  OF  MOISTURE 

ber  of  spines  on  the  swimming  feet,  but  none  of  these 
differences  seem  to  be  especially  connected  with  the  de- 
gree of  salinity."  Probably  the  Artemia  recently 
found  by  R.  T.  Giinther  *  inhabiting  Lake  Urmi  in 
Persia  in  such  enormous  numbers  is  only  another  local 
variety  of  A.  salina.  It  differs  from  this  species  in 
possessing  an  incompletely  segmented  abdomen,  in  the 
claspers  of  the  male  being  larger,  and  in  other  char- 
acters, but  Giinther  says  he  is  nevertheless  inclined  to 
agree  with  Packard  that  there  is  only  one  well-defined 
Old  World  species  of  Artemia,  viz.,  A.  salina. 

Schmankewitsch  also  changed  the  salinity  in  the  re- 
verse direction,  and  gradually  diluted  the  salt  water 
containing  some  A.  salina  till  it  finally  became  per- 
fectly fresh.  The  Crustaceans,  which  had  gone 
through  several  generations  during  the  process,  had 
meanwhile  so  changed  their  character  that  in  Schman- 
kewitsch's  opinion  they  now  resembled  the  form  of  the 
genus  Branchipus.  Thus  the  last  segment  of  the  post- 
abdomen  became  divided  into  two  segments,  and 
Schmankewitsch  maintained  that  this  division  of  the 
segment  is  the  only  structural  character  really  differ- 
entiating the  genus  Branchipus  from  Artemia.  How- 
ever, Glaus  f  has  shown  that  there  are  many  other  points 
of  difference,  and  that  the  division  in  question  is  not  a 
structural  character  of  great  importance.  Also 
Branchipus  is  distinguished  by  the  sexual  characters  of 
its  males,  which  possess  no  structure  in  any  way  similar 
to  the  great  leaf-like  second  antennae  shown  by  the 
male  Artemia.  "We  must  conclude,  therefore,  that 

*  Journ.  Linn.  Soc.  (Zool.),  xxvii.  p.  395. 
f  Auz.  Ak.  Wiss.  Wien.,  p.  43,  1886. 


AND  OF  SALINITY.  275 

though  decrease  of  salinity  does  produce  distinct  struct- 
ural changes,  yet  Schmankewitsch  considerably  exag- 
gerated their  importance,  and  deduced  from  them  more 
than  he  had  any  justification  for. 

In  addition  to  Artemia,  Schmankewitsch  *  studied 
the  effect  of  salinity  on  several  other  Crustaceans  such 
as  Daphnia  rectirostris,  DapJinia  magna,  and  Branchi- 
pus  ferox.  He  found  that  in  their  case  also  consider- 
able structural  and  physiological  changes  were  brought 
about,  the  fresh-  and  salt-water  forms  differing,  in  his 
opinion,  by  characters  usually  held  to  be  specific. 

Equally  interesting  evidence  as  to  the  effect  of  grad- 
ual increase  of  salinity  has  been  obtained  by  Bateson  f 
in  the  case  of  the  common  cockle,  Cardium  edule.  This 
mollusc,  together  with  several  others,  is  present  in 
enormous  numbers  in  the  brackish  waters  of  the  Aral 
Sea.  The  waters  of  this  closed  basin  have  been  gradu- 
ally drying  up  and  receding,  but  the  area  left  exposed 
"  is  not  a  level  tract,  but  contains  three  considerable 
depressions,  called  respectively  Shumish  Kul,  Jaksi 
Klich,  and  Jaman  Klich.  .  .  These  depressions  re- 
mained, for  a  time,  as  isolated  lakes,  each  containing  a 
separate  sample  of  the  fauna  of  the  sea  living  in  it." 
As  they  gradually  dried  up,  becoming  salter  and  salter, 
the  character  of  the  shells  progressively  changed.  To 
determine  this  change,  samples  were  collected  at  vari- 
ous levels  in  the  lake  areas,  and  were  carefully  com- 
pared. On  the  western  shore  of  Shumish  Kul  there 
were  seven  very  definite  terraces  of  muddy  salt,  show- 
ing the  position  of  the  water  at  various  periods  during 
the  gradual  drying  up.  The  changes  produced  con- 

*  Ibid.  f  Phil  Trans.  1889,  B.  p.  297. 


276  THE  EFFECT  OF  MOISTURE 

sisted  in  (1)  a  diminution  in  the  thickness  of  the  shells, 
this  being  first  apparent  in  the  shells  of  the  third  ter- 
race. So  marked  was  this  change  that  the  shells  of  the 
seventh  or  lowest  terrace  were  almost  horny  and  semi- 
transparent,  and  their  weight  was  not  a  third  that  of 
shells  from  the  first  two  terraces;  (2)  a  diminution  in 
the  size  of  the  beak;  (3)  a  high  colouration  in  the 
shells.  This  change  occurred  almost  uniformly,  the 
shells  of  each  terrace  being  very  nearly  alike  in  texture, 
thickness,  and  degree  of  colouration;  (4)  grooves  be- 
tween the  ribs  appearing  on  the  inside  of  the  shell  as 
ridges  with  rectangular  faces;  (5)  a  great  diminution 
in  the  absolute  size  of  the  shells  on  the  lowest  terrace; 
(6)  an  increase  in  the  length  (greatest  antero-posterior 
dimension)  of  the  shells  in  proportion  to  their  breadth, 
this  ranging  from  the  average  ratio  of  1 :  .80  in  the  shells 
from  the  first  terrace  to  1:.725  in  shells  from  the 
seventh.  In  Jaksi  Klich  lake  the  shells  from  the 
lowest  and  most  saline  deposit  were  even  more  elon- 
gated, the  ratio  of  length  to  breadth  being  as  1:  .68  for 
samples  of  smaller  shells,  and  1:.66  for  samples  of 
larger  ones.  Those  from  the  lowest  deposit  of  Jaman 
Klich  showed  about  the  same  degree  of  elongation  as 
those  from  the  lowest  terrace  of  Shumish  Kul. 

It  was  very  noticeable  that  the  shells  of  each  sample, 
whether  from  a  separate  lake  or  only  from  a  particular 
terrace,  resembled  each  other  more  closely  than  they 
did  shells  from  one  of  the  other  lakes,  or  those  from 
another  terrace  in  the  same  lake,  as  at  Shumish  Kul. 

In  each  of  the  three  lakes  mentioned  (and  also  in  an 
entirely  distinct  locality,  the  lagoon  of  Abu  Kir  near  to 
Alexandria),  it  was  thus  found  that  shells  which  had 


AND  OF  SALINITY.  277 

lived  in  very  salt  water  had  become  like  each  other  in 
possessing  the  characters  of  thinness,  high  colour, 
small  beaks,  ribbing  on  the  inside,  and  great  relative 
length.  "  In  view  of  these  four  instances  of  similar 
variations  occurring  under  similar  conditions,"  says 
Bateson,  "  it  seems  almost  certain  that  these  condi- 
tions are  in  some  way  the  cause  of  the  variations."  In 
that  the  variations  in  the  quality,  texture,  and  colour  of 
the  shell  are  found  developed  to  nearly  the  same  degree 
in  all  the  individuals  of  successive  terraces,  Bateson 
considers  they  may  be  fairly  supposed  to  be  the  direct 
result  of  environmental  change;  but  the  quality  of  in- 
creased proportional  length  is  not  found  in  all  the  in- 
dividuals, and  hence  may  have  arisen  in  some  other 
way,  as  by  Natural  Selection  of  the  type  best  fitted  to 
live  in  the  altered  state. 

A  further  proof  of  Bateson's  view  is  afforded  by  the 
fact  that  when  the  salinity  was  altered  in  the  direction 
of  diminution, the  characters, of  the  shells  were  similarly 
changed  in  a  reverse  direction.  Thus,  as  already  men- 
tioned, the  cockles  from  the  very  saline  lake  of  Abu  Kir 
resembled  those  from  the  lakes  of  the  Aral  Sea,  but 
close  to  this  lake  are  three  small  areas  of  water,  the 
Kamleh  lakes,  of  which  the  water  is  now  quite  fresh 
(owing  to  their  receiving  waste  water  from  the  irriga- 
tions). One  of  these  lakes  contains  living  cockles,  and 
another  the  shells  of  extinct  ones.  Now  in  both  in- 
stances the  shells  are  thick  and  coarse  in  texture,  and 
comparatively  light-coloured.  However,  the  feature 
of  great  proportional  length  still  remains. 

Other  evidence  as  to  the  relation  between  salinity 
and  structure  in  molluscs  has  been  obtained  by  Gib- 


278  THE  EFFECT  OF  MOISTURE 

bons  *  for  certain  tropical  and  sub-tropical  species  of 
Littorina.  These  organisms  are  confined  to  more  or 
less  brackish  waters,  and  seem  incapable  of  living  in 
pure  salt  water.  Gibbons  says  he  has  "  met  with  three 
of  these  species,  and  in  each  case  they  have  been  dis- 
tinguished from  the  truly  marine  species  by  the  ex- 
treme (comparative)  thinness  of  their  shells,  and  by 
their  colouring  being  richer  and  more  varied;  they  are 
also  usually  more  elaborately  marked."  Thus  diminu- 
tion of  salinity  seemed  to  have  produced  thinness  of 
shell  in  the  species  as  a  whole,  but  within  their  own 
limits  it  was  found  that  the  reverse  relation  held,  and 
that,  as  in  Bateson's  observations,  the  shells  became 
thinner  as  the  water  they  lived  in  became  more  salt. 

In  marine  animals,  as  in  fresh-water  ones,  increase 
of  salinity  probably  tends  to  diminish  the  rate  of 
growth.  Decrease  of  salinity,  on  the  other  hand,  may 
have  the  reverse  effect,  and  within  certain  limits  actu- 
ally increase  the  growth  rate.  Thus  Loeb  t  determined 
the  rate  of  regeneration  of  decapitated  hydroid  polyps 
(Tubularia  mesembryanthemum)  placed  in  sea-water  of 
various  degrees  of  dilution  and  concentration,  seven  to 
nine  individuals  being  measured  at  each  concentration. 
His  results  are  reproduced  in  the  figure  given  below. 
Here  the  Specific  Gravity  of  the  water  is  represented 
along  the  abscissa,  and  the  amount  of  regeneration  by 
the  height  of  the  ordinates.  We  see  that  the  maximum 
rate  of  regeneration  took  place  in  water  of  Sp.  G. 
1.025,  or  in  very  considerably  diluted  sea-water  (the  Sp. 
G.  of  this  being  about  1.038).  At  this  dilution  the  re- 

*  Quart.  Journ.  Conch.,  i.  p.  339. 

t  "  Biological  Lectures  delivered  at  Wood's  Holl,"  1893,  p.  46. 


AND  OF  SALINITY. 


279 


generation  was  more  than  twice  as  fast  as  in  normal 
water,  but  with  further  dilution  it  rapidly  diminished, 
and  ceased  altogether  in  water  of  Sp.  Gr.  1.013. 

Somewhat  similar  results  to  these  were  obtained  by 
the  author  for  sea-urchin  plutei.  As  we  saw  in  the 
last  chapter,  the  actual  size  of  an  organism  is  probably 
affected  by  environment  in  a  similar  manner  to  the 
growth  rate,  and  the  author  found  that  these  plutei, 


i.fe 


ife" 


•.If — nb 

Concentrated  Sea  Water 


01  1.02 

Dilute  Sea  Water 


Specific  Gravity 
FIG.  24. — Effect  of  salinity  on  growth  of  Tubularia. 

allowed  to  develop  in  sea-water  of  various  concentra- 
tions, attained  a  greater  size  than  the  normal  when  kept 
in  moderately  diluted  water,  and  probably  a  slightly 
smaller  size  when  kept  in  concentrated  water.  The 
results  obtained  are  indicated  in  the  subjoined  figure, 
where  the  abscissae  represent  the  salinity  of  the 
water,  and  the  ordinates  the  average  percent- 
age variation  in  the  size  of  the  larvae  after  eight 
days'  growth,  as  compared  with  that  of  larvae  grown 
in  normal  sea-water.  The  salinity  of  normal  water 
was  taken  as  1000,  and  the  less  saline  waters  were 
*Phil.  Trans.  1895,  B.  p.  586. 


THE  EFFECT  OF  MOISTURE. 


obtained  by  diluting  respectively  950,  900  c.  c.,  etc., 
of  water  to  a  litre;  the  more  saline  by  concentrat- 
ing 1050,  1100  c.  c.,  etc.,  to  a  litre.  The  maximum 
effect  on  size  was  produced  by  a  solution  containing 
50  c.  c.  of  fresh  water  per  litre,  the  increase  amount- 
ing to  15  per  cent.  With  further  dilution,  the  favour- 
able influence  became  less  and  less,  till,  with  water  con- 


+20 


415 


-1: 


850 


900 


950          1000          1050 
Salinity  o^  gea-water 


1100 


1150 


1200 


FIG.  25.  —  Effect  of  salinity  on  size  of  sea-urchin  larvae. 


taming  150  c.  c.  per  litre,  it  was  negative.  Thus  the 
optimum  salinity  is  for  a  much  less  diluted  water  than 
in  the  case  of  the  tubularians.  The  present  results  also 
differ  from  these  latter  in  that  more  concentrated 
waters  have  exceedingly  little  effect  on  the  size  of  the 
larvae. 


CHAPTER  IX. 

THE  EFFECT  OF  FOOD  AND  OF  PRODUCTS  OF 
METABOLISM. 

Effect  of  artificial  manures  on  growth  of  crops— Effect  of  nutrition 
on  plant  variation — Development  of  bees  and  of  aphides  in  rela- 
tion to  food — Influence  of  nature  of  food  on  wing  markings  of 
certain  Lepidoptera— Dependence  of  colour  of  larvae  on  plant  pig- 
ments— Influence  of  food  on  growth  of  tadpoles — Plumage  of  cer- 
tain birds  altered  by  abnormal  diet — Quality  of  food  influences 
organs  of  digestion — Every  organism  probably  has  specific  metab- 
olism, which  has  especially  adverse  action  on  its  own  growth — 
Products  of  metabolism  may  stimulate  growth — Effects  of  small 
quantities  of  urea,  uric  acid,  and  ammonium  salts — Influence  of 
volume  and  of  surface  area  of  water  on  growth  of  pond  snail — 
Influence  of  surface  area  on  growth  of  tadpole — Effects  of  increas- 
ing quantities  of  metabolic  products  on  characters  of  a  snail,  and 
of  a  Crustacean. 

DARWIN  records  *  that  Andrew  Knight  was  of  the 
opinion  that  "  of  all  the  causes  which  induce  variability, 
excess  of  food,  whether  or  not  changed  in  nature,  is 
probably  the  most  powerful."  Darwin  himself,  more- 
over, was  inclined  to  accept  this  view  of  the  potency  of 
food  as  probable.  That  changes  in  the  amount  and  the 
quality  of  the  food  available  for  an  organism  during  its 
growth  must  of  necessity  exert  an  important  influence 
on  the  course  of  that  growth,  and  presumably,  there- 
fore, on  the  final  limits  of  its  attainment,  is  sufficiently 
obvious  both  from  one's  own  everyday  experience,  and 

*  "  Animals  and  Plants,"  vol.  ii.  p.  244. 


THE  EFFECT  OF  FOOD 


from  a  simple  recognition  of  the  relation  between  cause 
and  effect.  Growth  can  only  take  place  at  the  expense 
of  food  material,  and  unless  this  is  always  more  than 
sufficient  for  the  needs  of  the  organism,  the  rate  of 
growth  must  be  dependent  upon  it. 

In  spite  of  the  importance  of  changes  in  feeding  as  a 
source  of  variation,  the  number  of  direct  and  une- 
quivocal experiments  made  upon  the  subject  is  compara- 
tively small,  for  most  of  them  are  complicated  by  simul- 
taneous changes  in  other  conditions  as  well.  Upon 
members  of  the  vegetable  kingdom,  the  experiments 
made  by  Lawes  and  Gilbert  *  at  Kothampsted  during 
the  last  fifty  years  afford  most  valuable  evidence. 
These  concern  the  effect  of  various  manures  on  the 
growth  of  barley,  wheat,  and  various  leguminous  plants. 
In  the  accompanying  table  are  given  the  average 


200  LB.  AM- 

275  LB.  SO- 

NO NITRO- 

MONIUM 

DIUM    NI- 

1000        LB. 

ADDITIONS  TO  SOIL. 

GEN  OTIS 
MANURE. 

SALTS,    = 
43  LB.  NI- 

TRATE, = 
43  LB.  NI- 

RAPE CAKE 
=  49  LB. 

TROGEN. 

TROGEN. 

NITROGEN. 

Without  mineral  manure, 

16.5 

29 

32.7 

41.2 

Superphosphate, 
Potassium,  sodium  and  magne- 
sium sulphates, 
Superphosphate  and  K.  Na.  and 

21.7 
18 

42.7 
31.4 

45.7 
33.5 

43.4 
39.5 

Mg.  sulphates, 

22.4 

43.5 

45.5 

43.2 

amounts  of  barley  grain  (in  bushels  per  acre)  obtained 
each  year  from  soils  treated  in  various  ways.  These 
observations  were  carried  on  for  forty  years  in  succes- 
sion (1852-91)  upon  the  same  land,  and  so  represent 
strictly  average  results,  from  which  errors  due  to  varia- 

*"The  Rothampsted  Experiments,"  p.  78,  Edinburgh  and  Lon- 
don, 1895. 


AND  OF  PRODUCTS  OF  METABOLISM.         283 

tions  of  season,  and  other  causes,  are  practically  elimi- 
nated. 

From  this  table  we  see  that  the  average  yield  from 
land  left  entirely  without  manure  was  16.5  bushels  of 
grain.  On  adding  various  manures,  all  of  which  con- 
tained about  the  same  weight  of  combined  nitrogen,  the 
yield  of  grain  was  doubled,  or,  in  the  case  of  rape-cake 
manure,  increased  to  two  and  a  half  times  the  amount. 
The  addition  of  various  inorganic  salts  to  the  soil  also 
had  a  favourable  effect,  though  to  nothing  like  the  same 
degree  as  that  of  the  nitrogenous  manures.  Thus  we  see 
that,  when  no  nitrogenous  manure  whatever  is  present, 
the  addition  of  superphosphates  increases  the  yield  by 
32  per  cent.;  of  potassium,  sodium  and  magnesium  sul- 
phates by  9  per  cent. ;  and  of  both  superphosphates  and 
these  sulphates,  by  36  per  cent.  When  the  nitrogen  is 
added  as  ammonium  salts  or  nitrates,  then  combinations 
of  nitrogenous  and  mineral  manures  give  a  very  much 
better  yield  than  the  nitrogenous  manure  alone,  but 
when  it  is  added  as  rape  cake,  the  growth  of  the  crop 
has  already  been  so  much  increased  that  the  further 
addition  of  mineral  salts  effects  but  little.  Yet  even  the 
highest  of  the  numbers  in  this  table  does  not  represent 
the  maximum  amount  of  growth  of  which  the  barley  is 
capable,  for  a  soil  treated  with  farm-house  manure,  and 
no  additional  mineral  salts,  yielded  on  an  average  48.6 
bushels  per  acre.  In  all  these  experiments  the  yield  of 
straw  was  increased  in  more  or  less  similar  proportions 
to  the  yield  of  grain,  and  hence  we  may  conclude  that 
the  growth  of  a  plant  in  normal  soil  can  be  very  nearly 
trebled  if  only  favourable  enough  conditions  are 
afforded  it. 


284  THE  EFFECT  OF  FOOD 

Somewhat  similar  results  to  these  were  obtained  by 
Lawes  and  Gilbert  for  wheat,  bean,  clover,  and  other 
crops,  but  it  is  deemed  unnecessary  to  reproduce  them 
here. 

Most  striking  evidence  as  to  the  influence  of  nutri- 
tion on  variations  has  been  obtained  by  De  Tries.* 
When  carrying  out  his  artificial  selection  experiments 
on  five-leaved  clover,  he  found  that  in  one  series  of  ob- 
servations seeds  from  some  plants  grown  in  a  poor  soil 
yielded  39  per  cent,  of  3-leaved,  and  48  per  cent,  of  5- 
to  7-leaved  clover.  Those  from  plants  of  the  same 
stock  which  had  been  grown  in  a  rich  soil,  however, 
gave  only  14  per  cent,  of  3-leaved,  and  73  per  cent,  of 
5-  to  7-leaved  clover. 

The  effect  of  nutrition  on  Ranunculus  bulbosus 
(Crowfoot)  was  almost  as  striking. f  Wild  flowers  col- 
lected near  Hilversum  were  found  by  De  Vries  to  have 
the  following  frequencies  of  distribution  in  the  numbers 
of  their  petals : 

Number  of  petals,          5        6       7       8       9        10        11 
657      41       11        2        4          0          2 

In  the  autumn  of  1887  De  Vries  planted  some  of 
these  plants  in  his  culture  garden,  where  they  bloomed 
the  following  year.  Owing,  presumably,  to  the  better 
nutrition,  the  proportion  of  flowers  with  more  than  five 
petals  was  considerably  increased: 

Number  of  petals,  5         6         7        8       9        10 

133        55        23        7        2          2 

The  seed  from  the  many-petalled  flowers  was  collected 

*"  Die  Mutationstheorie,"  p.  448. 

fBer.  d.  deutsch.  Bot.  Ges.,  xii.  p.  197,  1894. 


AND  OF  PRODUCTS  OF  METABOLISM         285 

and  sown  through  two  seasons,  and  from  the  seed  then 
obtained  372  plants  were  grown.  Some  of  these  ger- 
minated early,  and  so  developed  under  less  favourable 
conditions  than  the  others.  As  will  be  seen  from  the 
accompanying  figures,  these  early  plants  had  9  petalled 
flowers  occurring  the  most  frequently,  whilst  the  later 
ones  had  10  petalled  flowers;  i.  e.,  flowers  with  twice 
the  original  number  of  petals: 

Number  of  petals,  5  6  7  8  9  10  11  12  13  14  15  16—81 
Early  plants,  409  532  638  690  764  599  414  212  80  29  18  20 

Late  plants,  40     52    126    165    204  215    177    104     35     8       4  0 

The  somewhat  unexpected  results  obtained  by  Mac- 
Leod *  with  Ficaria  ranunculoides  may  also  be  attrib- 
uted, at  least  in  some  degree,  to  the  effects  of  nutrition. 
MacLeod  determined  the  numbers  of  stamens  and  of 
pistils  in  the  flowers  borne  by  a  number  of  plants  at  the 
beginning  of  the  flowering  season,  and  again  in  the 
flowers  borne  by  the  same  plants  at  the  end  of  the  sea- 
son. The  early  flowers  had  on  an  average  26.73 
stamens  and  17.45  pistils,  whilst  the  late  ones  had  only 
17.86  stamens,  and  12.15  pistils.  Also  the  variability 
in  the  number  of  stamens  and  of  pistils  was  very  dif- 
ferent in  the  two  cases,  the  coefficients  of  variation 
being  respectively  14.1  and  22.3  per  cent,  in  the  early 
flowers,  and  18.5  and  27.9  per  cent,  in  the  late  ones. 
The  method  adopted  by  MacLeod  for  estimating  the  cor- 
relation between  the  numbers  of  stamens  and  of  pistils 
is  erroneous,  so  Professor  Weldon  has  recalculated  the 
constants.!  He  finds  that  MacLeod's  figures  indicate 
the  correlation  to  be  much  less  in  the  early  than  in  the 

*  Botanisch.  Jaarboek.,  xi.,  1897. 
fBiometrika,  i.  p.  125,1901. 


286  THE  EFFECT  OF  FOOD 

late  flowers  (the  r  being  respectively  .51  and  .75  in  the 
two  cases).  As  Professor  Weldon  remarks,  these  re- 
sults "  provide  a  most  valuable  lesson  as  to  the  possible 
danger  of  asserting  that  such  differences  are  significant 
of  local  races." 

By  observations  upon  the  growth  of  seedlings  placed 
in  various  solutions,  it  has  long  been  known  that  normal 
growth  is  possible  only  if  various  inorganic  salts  are 
present.  There  must  be  nitrogen  in  the  form  of 
nitrates  or  ammonium  salts,  sulphur  in  the  form  of  sul- 
phates, phosphorus  as  phosphates,  chlorine  as  chlorides, 
and  the  metals  sodium,  potassium,  magnesium,  calcium, 
and  iron  in  solution  as  salts.  The  absence  of  any  one 
of  these  substances  speedily  inhibits  normal  growth;  as 
soon,  in  fact,  as  the  seedling  has  exhausted  the  small 
quantity  of  it  stored  up  within  itself.  For  instance, 
plants  grown  in  solutions  containing  no  iron  soon  show 
a  sickly  appearance;  "the  leaves  are  no  longer  green, 
but  white,  and  microscopic  examination  of  them  shows 
that  abnormal  chlorophyll  bodies,  or  none  at  all,  are 
present  in  their  cells.  If  we  add  to  the  food  solution 
a  few  drops  of  dilute  ferric  chloride  solution,  the  pre- 
viously white  leaves  become  green  in  two  or  three  days, 
and  the  growth  of  the  plants  now  proceeds  normally."  * 
It  follows,  therefore,  that  if  the  absence  of  these  vari- 
ous substances  stops  growth  altogether,  a  deficiency  in 
them  must  produce  diminished  or  abnormal  growth, 
and  so  lead  to  the  production  of  variations. 

With  members  of  the  Animal  Kingdom,  variations  in 
the  inorganic  salts  of  the  food  may  also  be  a  source  of 

*  Quoted  from  Detmer's  "  Practical  Plant  Physiology,"  p.  84. 


AND  OF  PRODUCTS  OF  METABOLISM.        287 

variation.  Thus  Cooke  *  states  that  "  a  deficiency  of 
lime  in  the  composition  of  the  soil  of  any  particular 
locality  produces  very  marked  effects  upon  the  Mol- 
lusca  which  inhabit  it;  they  become  small  and  very  thin, 
occasionally  almost  transparent.  The  well-known  var. 
tennis  of  Helix  aspersa  occurs  on  downs  in  the  Channel 
Islands  where  calcareous  material  is  scarce.  For  simi- 
lar reasons,  II.  arbustorum  develops  a  var.  fusca,  which 
is  depressed,  very  thin,  and  transparent,  at  Scilly  and 
also  at  Lunna  I.,  E.  Zetland." 

However,  in  animal  development  the  supply  of  in- 
organic salts  is  almost  always  more  than  sufficient  for 
the  needs  of  the  organism,  and  such  variations  as  are 
produced  are  due  chiefly  to  the  organic  constituents  of 
the  food.  Among  invertebrate  animals,  our  knowl- 
edge of  the  direct  influence  of  food  is  almost  confined  to 
certain  of  the  Insecta.  In  the  case  of  bees,  it  has  been 
known  for  a  very  long  time  that  the  quality  and  quan- 
tity of  the  food  supplied  to  the  larvae  determines 
whether  the  reproductive  organs  shall  undergo  their 
full  development,  and  produce  fertile  queens,  or  remain 
undeveloped,  and  so  produce  non-fertile  working  fe- 
males. According  to  A.  von  Planta,  the  diet  of  the 
queen  larvae  contains  twice  as  much  fatty  material  as 
that  of  the  worker  s.f  Again,  Eimer  has  pointed  out 
that  in  the  case  of  the  humble  bee,  the  first  brood  of 
ova,  laid  in  the  spring,  get  only  a  scanty  supply  of  nutri- 
ment, and  develop  into  small  females,  which  are  fertile 
though  they  can  only  produce  drones.  The  next  brood 

*  Vol.  iii.,  "  Cambridge  Natural  History,"  p.  89. 

f  Quoted  from  Creddes  and  Thomson's  "  Evolution  of  Sex,"  p.  43, 


288  THE  EFFECT  OF  FOOD 

born  obtain  more  nourishment,  and  develop  into  larger 
females,  which  are  capable  of  occasionally  producing 
females,  as  well  as  drones.  Finally  the  future  queens, 
which  obtain  a  still  richer  diet,  are  born.  The  deter- 
mination of  sex  seems  to  be  dependent  on  nutrition  also 
in  aphides  or  plant-lice.  Thus  "  during  the  summer 
months,  with  favourable  temperature  and  abundant 
food,  the  aphides  produce  parthenogenetically  genera- 
tion  after  generation  of  females.  The  advent  of  au- 
tumn, however,  with  its  attendant  cold  and  scarcity  of 
food,  brings  about  the  birth  of  males,  and  the  conse- 
quent recurrence  of  strictly  sexual  reproduction."  * 
In  this  instance,  therefore,  the  effect  of  nutrition  is 
bound  up  with  that  of  temperature,  and  there  are  no 
data  to  show  whether  either  of  these  conditions  could 
produce  the  effect  if  acting  alone. 

Upon  the  Lepidoptera  the  effects  of  various  foods 
have  been  tested  in  a  considerable  number  of  instances. 
Observations  were  made  by  Gr.  Koch  f  in  Germany  as 
long  ago  as  1832.  By  feeding  the  caterpillars  of  Che- 
Ionia  Jiebe  with  different  plants,  he  obtained  specimens 
which  were  either  fiery  or  dull  red  on  the  under  wings, 
and  which  varied  in  the  extent  of  black  marking  and 
white  ground.  In  the  case  of  Euprepia  caja  (Common 
tiger  moth)  it  is  known,  Koch  says,  "  that  when  the 
caterpillars  are  fed  from  their  hatching  to  their  meta- 
morphosis with  leaves  of  lettuce  or  deadly  nightshade, 
not  one  of  the  imagines  produced  resembles  the  origi- 
nal form;  when  the  insects  have  been  fed  on  lettuce, 
the  white  ground-colour  of  the  wings  predominates; 

*Ibid.t  p.  46. 

fEimer,  "  Organic  Evolution,"  p.  149. 


AND  OF  PRODUCTS  OF  METABOLISM.    289 

when  fed  on  deadly  nightshade  the  brown  markings  of 
the  upper  wings  often  coalesce  and  the  white  vanishes; 
in  like  manner  the  blue  markings  on  the  lower  wings 
fuse  together  and  displace  the  orange-yellow  ground 
colour."  * 

A  careful  series  of  observations  upon  various  moths, 
extending  over  some  ten  years,  has  been  made  by  Greg- 
son,  f  His  results  may  be  tabulated  as  follows: 

Pygcera  bucepTiala  (Buff  Tip)  is  finer  and  darker  when  fed  upon 

sycamore. 
Xylophasia  polyodon  (Dark  Arches)  is  dark,  sometimes  black,  when 

fed  upon  heather. 

Hadena  adusta  (Dark  Brocade)  is  darker  when  fed  upon  heath. 
Acronycta  menanthydis  (Light  Knot-Grass)  when  fed  on  sallow,  often 

produces   var.    A.  salycis;   fed  on  heath,  produces  light 

specimens. 
Hybernia  defoliaria  (Mottled  Umber)  is  beautifully  marked  when  fed 

upon  birch;  but  on  elm  gives  dull-coloured  forms,  almost 

without  markings. 
Eupithecia  venosaria  (Netted  Pug)  fed  on  inflated  catchfly  is  almost 

white;  on  shore  catchfly  is  much  larger  and  almost  lead 

colour. 
Noctua  f  estiva  (Engrailed  Clay)  fed  on  thorn  is  rich  red  and  well 

marked:    on  grasses  is  light   yellowish,   and  rarely  well 

marked. 
Noctua  triangulum  (Double-Square  Spot)  fed  on  thorn  is  dark:  fed 

on  low  plants  is  light. 
Abraxas  grossulariata  (Magpie)  fed  on  red  currant  is  light;  on  black- 

thorn is  darker;    on  bullace  or  wild  plum  is  darker  still, 

the  white  sometimes  becoming  yellow. 

The  following  case,  recorded  by  the  late  Mr.  New- 
man^ is  of  especial  interest  in  that  it  occurred  under 
natural  conditions.  The  larva  of  V.  polychloros  (Large 


.,  p.  151. 
fThe  Zoologist,  p.  7903,  1862. 
j  The  Entomologist,  vi.  p.  88,  1872. 


290  THE  EFFECT  OF  FOOD 

Tortoiseshell)  usually  feeds  upon  elm,  but  that  of  V. 
urticce  (Small  Tortoiseshell)  upon  nettles.  Some 
larvae  were  found  by  Mr.  J.  A.  Tawell  feeding  upon 
nettles,  and  so  were  considered  to  be  those  of  V.  urtiwz. 
They  were  accordingly  kept  on  nettles,  but  to  his  sur- 
prise developed  into  V.  polychloros  imagines.  "  These 
specimens,"  records  Newman,  to  whom  they  were 
shown,  "  have  a  wonderful  similarity  to  urticce,  which 
they  do  not  at  all  exceed  in  size;  still  the  colour  is 
nearer  to  that  of  polychloros  than  that  of  urtictz"  The 
effect  of  abnormal  food  on  Melitcea  artemis  (Greasy 
Fritillary)  has  been  noticed  by  H.  Gross.*  By  feeding 
the  larvae  on  honeysuckle,  a  series  of  very  dark  imagines 
was  obtained,  which  differed  both  in  size  and  colouring 
from  all  other  specimens  known  to  him,  though  these 
had  been  derived  from  very  varied  localities  in  Eng- 
land, Ireland,  and  Scotland.  Again,  the  quantity  of 
the  food  supplied  may  have  as  considerable  an  effect  as 
the  quality.  By  mistake  some  V.  io  (Peacock  butter- 
fly) larvae,  captured  by  Mr.  R.  Cox,f  were  left  for  sev- 
eral days  without  fresh  food,  and  all  the  dead  leaves  and 
stalks  were  devoured.  Nearly  all  the  imagines  ob- 
tained from  them  were  rather  small,  but  they  also 
varied  much  in  the  intensity  of  their  colouring,  two 
specimens  being  very  much  darker  than  usual,  with  the 
yellow  in  the  costal  spot  and  ocellus  much  reduced.  It 
seems  to  me,  however,  that  probably  these  changes  were 
due  rather  to  the  abnormal  food  devoured  by  the  larvae 
than  to  the  actual  lack  of  food. 

As  regards  the  larvae  of  Lepidoptera,  the  obvious  re- 

*The  Entomologist,  vii.  p.  203,  1874, 
f  The  Entomologist,  ix.  p.  58, 


AND  OF  PRODUCTS  OF  METABOLISM.        291 

lation  between  their  colour  and  that  of  their  food 
seems  to  show  that  the  one  is  directly  dependent 
on  the  other.  Meldola  *  accounted  for  it  by  sup- 
posing that  the  larvae  had  been  rendered  transparent 
by  Natural  Selection,  whereby  the  colour  of  the  vege- 
table food  eaten  was  itself  enabled  to  give  the  colour  to 
the  larvae.  Poulton  f  has  shown  that  the  colours  of  the 
larvae  are  due  partly  to  the  pigments  proper  to  the 
larva,  and  partly  to  the  pigments  derived  from  the  food 
plants.  These  pigments  undergo  some  modification  in 
the  tissues,  but  Poulton  states  that  as  far  as  he  has  in- 
vestigated the  subject  "  all  green  colouration  without 
exception  is  due  to  chlorophyll ;  while  nearly  all  yellows 
are  due  to  xanthophyll."  The  chlorophyll,  or  some 
modification  of  it,  tinges  the  blood  of  the  larvae,  the 
green  colour  of  which  is  often  due  to  this  cause  alone. 

From  these  observations,  therefore,  it  follows  that  a 
change  of  food  may  also  effect  a  change  of  colouration. 
That  this  is  so  is  strikingly  shown  by  some  other  obser- 
vations by  Poulton.  J  Obtaining  a  large  number  of 
larvae  of  Tryphcena  pronuba  (Common  yellow  under- 
wing)  from  the  same  batch  of  eggs,  he  split  them  up 
into  three  groups.  One  he  fed  on  the  white  midribs  of 
the  cabbage,  from  which  the  yellow  blade  had  been 
carefully  removed  with  scissors.  These  larvae  remained 
almost  white  at  first,  and  afterwards  showed  a  moderate 
amount  of  black  pigmentation.  The  other  two  groups 
of  larvae  he  fed  respectively  on  the  yellow  etiolated 
leaves  from  the  heart  of  the  cabbage,  and  upon  the  deep 

*  Proc.  ZoSl.  Soc.  1873,  p.  155. 
fProc.  Hoy.  Soc.,  xxxviii.  p.  269,  1885. 
jProc.  Roy.  Soc.,  liv.  p.  417,  1893. 


292 


THE  EFFECT  OF  FOOD 


green  external  leaves.  These  larvae,  however,  were  all 
of  a  bright  green  or  brown  colour.  Hence  it  would 
seem  that  both  etiolin  and  chlorophyll  are  capable  of 
being  transformed  into  a  larval  colouring  matter,  which 
may  be  either  green  or  brown. 

As  regards  the  effects  of  feeding  among  vertebrate 
animals,  a  careful  series  of  experiments  upon  tadpoles 
(Eana  esculenta)  has  been  made  by  Yung.*  The  tad- 
poles were  all  derived  from  the  same  batch  of  eggs,  and 
were  placed,  in  groups  of  fifty,  in  six  similar  jars  of 
water.  All  the  conditions  of  development  such  as 
light,  temperature,  and  frequency  of  change  of  water, 
were  identical,  the  food  alone  being  varied.  The  kinds 
of  food  supplied,  and  the  average  size  attained  by  the 
tadpoles  after  42  days'  development  (three  being  meas- 
ured in  each  case),  are  given  in  the  accompanying  table: 


NATURE  OF  POOD. 


0  W 


ii 


o  §  S'fc 

l!« 

go*§ 

Sgw* 

1B1 

lr'§ 


go 


Length  of  tadpole 
Breadth  of  tadpole 


18.3 
4.2 


23.2 
5.0 


26.0 
5.8 


33.0 
6.6 


38.0 

8.8 


43.5 

9.2 


Per  cent,  of  frogs 
after  58  days. 


14 


20 


48 


66 


Here  we  see  that  the  purely  vegetable  diet  acted  least 
favourably,  and  the  beef  diet  the  most  favourably. 
Egg  yolk  did  not  answer  so  well  as  coagulated  egg  al- 
bumen, but  better  than  uncoagulated  albumen.  From 


*  Arch,  de  ZoSlogie  Exper.,  1883,  p.  3. 


AND  OF  PRODUCTS  OF  METABOLISM.         293 

the  bottom  line  of  the  table  we  see  that,  58  days  after 
the  beginning  of  the  experiment,  none  of  the  50  tad- 
poles fed  on  plants  and  on  liquid  egg  albumen  were  sur- 
viving; but  of  those  fed  on  fish  and  on  beef,  respect- 
ively 48  and  66  per  cent,  were  alive,  and  had  under- 
gone their  metamorphosis  into  frogs. 

The  effects  of  certain  foods  on  the  plumage  of  birds 
is  well  known  to  bird  fanciers.  Thus  hemp  seed  causes 
bull-finches  and  certain  other  birds  to  become  black. 
Cayenne  pepper,  mixed  with  the  food,,  changes  the  yel- 
low colour  to  an  orange  red.  This  colour  change  can 
only  be  effected  by  feeding  the  very  young  birds;  with 
adults  there  is  no  effect  whatever.  Sauermann  *  found 
that  all  races  are  not  equally  susceptible  to  the  abnor- 
mal diet,  some  being  changed  to  a  crimson,  others  to  a 
beautiful  orange,  whilst  others  remain  absolutely  un- 
affected. He  found  also  that  canaries  are  not  alone  in 
their  susceptibility,  for  on  feeding  some  white  Italian 
fowls,  eight  weeks  old,  with  the  pepper,  orange  stripes 
appeared  on  the  breast  feathers  of  one  of  them  after 
ten  days.  Later  on,  the  whole  body  was  covered  with 
mixed  white  and  orange  feathers,  and  the  breast  had  be- 
come red.  One  other  fowl  also  developed  a  red  breast, 
but  the  remaining  ten  showed  no  change  whatever. 
The  doses  of  Cayenne  pepper  given  were  enormous  (50 
gm.  daily),  so  that  the  conditions  were  absolutely  un- 
natural. 

More  remarkable  than  these  observations  are  the 
facts  ascertained  by  A.  R.  Wallace,  and  communicated 
by  him  to  Darwin,  f  Thus  he  states  that  "  the  natives 

*  Archiv  f.  Anatomic  u.  physiol.  Physiol.  Abtheil.,  p.  543,  1889. 
f  "  Animals  and  Plants,"  ii.  p.  269. 


294  THE  EFFECT  OF  FOOD 

of  the  Amazonian  region  feed  the  common  green  par- 
rot (Chrysotis  f estiva)  with  the  fat  of  large  Siluroid 
fishes,  and  the  birds  thus  treated  become  beautifully 
variegated  with  red  and  yellow  feathers.  In  the  Ma- 
layan archipelago,  the  natives  of  Gilolo  alter  in  an 
analogous  manner  the  colours  of  another  parrot, 
namely,  the  Lorius  garrulus,  and  thus  produce  the  Lori 
rajah  or  King  Lory." 

As  regards  mammals,  it  is  asserted  by  Nathusius  * 
that  if  rich  and  abundant  food  be  supplied  to  young 
pigs,  it  has  the  direct  effect  of  producing  a  broader  and 
shorter  head.  Poor  food,  on  the  contrary,  produces  a 
longer  and  narrower  head,  or  a  tendency  towards  the 
characters  of  the  wild  boar.  Again,  Krockerf  has 
shown  that  the  amount  of  wool  yielded  by  sheep  is 
greatly  influenced  by  the  quantity  of  food.  The  fol- 
lowing are  the  weights  of  wool  yielded  per  day  by  sheep 
weighing  in  aggregate  1000  kilograms  : 

DIET.  KILOGRAMS  OP  WOOL. 

Scanty  winter  food, 69 

Plenty  of  hay, 87 

Good  pasture,          .......        .96 

Fattening  process, 1.08—1.24 

The  quality  of  the  food  may  considerably  affect  the 
organs  of  digestion.  Thus  Cuvier  $  found  that  in  the 
wild  boar  the  length  of  the  intestines  is  to  that  of  the 
body  as  9  to  1,  but  in  the  common  domestic  boar  it  is  as 
13.5  to  1.  It  is,  of  course,  impossible  to  say  for  certain 
whether  this  increased  length  was  the  direct  result  of  a 
more  vegetable  diet,  but  it  seems  highly  probable  that 

*  "  Schweineschadel,"  p.  99;  also  "  Animals  and  Plants,"  i.  p.  75. 
f  De  Varigny's  "Experimental  Evolution,"  p.  90. 
\  "  Animals  and  Plants,"  i.  p.  77. 


AND  OF  PRODUCTS  OF  METABOLISM.         295 

this  was  so,  at  least  in  part.  The  observations  which 
have  been  made  from  time  to  time  as  to  the  effects  of 
various  kinds  of  food  on  the  thickness  of  the  stomach 
wall,  are,  however,  free  from  all  such  doubt.  The 
change  produced  must  evidently  be  the  direct  result  of 
the  altered  diet.  Thus  John  Hunter  observed  a  most 
marked  thickening  and  hardening  in  the  stomach  of  a 
gull  (Larus  tridadylus)  which  had  been  fed  for  a  year 
on  grain.  It  is  stated  by  Dr.  Edmondston  that  a  similar 
change  takes  place  under  natural  conditions  every  year 
in  the  stomach  of  the  common  Herring  gull  (Larus 
argentatus).  Thus  in  the  Shetland  Islands  this  bird 
feeds  in  the  winter  on  fish,  but  in  the  summer  fre- 
quents the  cornfields  and  feeds  on  grain.  Dr.  Edmond- 
ston has  also  noticed  a  somewhat  similar  change  in  the 
stomach  of  a  raven  which  had  been  fed  for  a  long  time 
on  vegetable  food.  Again,  Menetries  found  that  in  an 
owl  (Strix  grallaria)  the  effect  of  vegetable  diet  was  to 
change  the  form  of  the  stomach,  and  make  the  inner 
coat  leathery.* 

The  converse  experiment  of  feeding  graminivorous 
birds  on  a  flesh  diet  has  been  made  by  Dr.  Holmgren. 
By  feeding  pigeons  on  meat  for  a  considerable  time,  he 
found  that  the  gizzard  gradually  acquired  the  qualities 
of  a  carnivorous  stomach.  Again,  Delage  f  fed  a  fowl 
for  three  years  on  meat,  and  found  that  the  muscular 
substance  of  its  gizzard  was  considerably  decreased. 
All  these  results,  though  apparently  so  unequivocal, 
have  not  passed  unchallenged;  for  G.  Brandes,J  who 

*  Vide  "  Animals  and  Plants,"  ii.  p.  292. 
f  L'Annee  Biologique,  1896,  p.  468. 
j  Biol.  Centralblatt,  xvi.  p.  825. 


296  THE  EFFECT  OF  FOOD 

fed  both  flesh-feeding  birds  on  grain,  and  grain-feeders 
on  flesh,  states  that  he  was  unable  to  trace  any  adapta- 
tion to  the  altered  conditions  in  either  case. 

The  Effects  of  Products  of  Metabolism.  That  organ- 
isms react  on  each  othe*r  has  long  been  recognised.  The 
interdependence  is  especially  obvious  in  the  case  of 
parasite  and  host ;  but  reflection  will  show,  I  think,  that 
the  interaction  is  of  much  wider  scope  than  is  included 
in  such  self-evident  cases  as  these.  In  any  given 
volume  of  water,  or  any  given  area  of  land,  every  ani- 
mal and  every  vegetable  organism  may  to  some  extent 
affect  the  well-being  of  every  other  organism,  both  ani- 
mal and  vegetable.  The  animal  does  this  largely 
through  the  agency  of  its  own  specific  metabolism,  or 
through  the  specific  products  of  excretion  which,  com- 
ing into  contact  with  the  other  organisms,  in  turn  affect 
them.  That  every  species  of  animal  does  possess  a 
specific  metabolism  is,  perhaps,  scarcely  what  one  would 
on  a  priori  grounds  expect;  but  the  observations  made 
by  the  author  *  tend  to  prove  that  such  is  actually  the 
case.  These  observations  chiefly  concern  Echinoids, 
both  adult  forms  and  plutei,  but  more  especially  the  al- 
ready so  frequently  mentioned  plutei  of  Strongylo- 
centrotus. 

On  allowing  the  fertilised  ova  of  Strongylocentrotus 
or  of  Echinus  microtuberculatus  to  develop  in  water  in 
which  another  batcfi  of  larvae  (Strongylocentrotus, 
Spharechinus  or  Echinus)  had  already  been  developing 
for  8  to  12  days,  but  from  which  they  had  been  re- 
moved by  filtration,  it  was  found  that  in  every  case  they 

*  Vide  Mittheilungen  a.  d.  Zool.  Stat.  z.  Neapel.,  Bd.  xiii.  p.  389 
et  seq. 


AND  OF  PRODUCTS  OF  METABOLISM.         297 


were  diminished  in  size.  In  ten  experiments  the  aver- 
age diminution  was  7.1  per  cent.  It  was  concluded, 
therefore,  that  the  first  batch  of  larvae  had  excreted 
some  products  of  metabolism  into  the  water  which  had 
adversely  affected  the  growth  of  the  second  batch. 
Other  observations*  showed  that  the  growth  of  larvae 
may  be  affected  by  their  own  metabolic  products.  Thus 
it  was  found  that  the  arm  lengths  of  the  larvse  became 
smaller  and  smaller  the  larger  the  number  of  larvae  al- 
lowed to  develop  together  in  a  given  volume  of  water. 
In  the  accompanying  table  are  given  the  mean  results 
of  159  sets  of  measurements,  each  on  the  anal  and  oral 
arm  lengths  of  50  larvae. 


NUMBER  OP 

MEAN 

MEAN 

DIFFERENT 

NUMBER  OF  LARV^J  PER  LITRE. 

LENGTH  Of 

LENGTH  OF 

OBSERVATIONS. 

ANAL  ARM. 

ORAL  ARM. 

37 

Under  1500 

121.2 

118.4 

32 

1500  to  3500 

114.0 

110.5 

21 

3500  to  6000 

105.8 

101.0 

34 

6000  to  11,000 

102.9 

99.4 

27 

11,000  to  20,000 

95.7 

94.2 

6 

20,000  to  30,000 

85.5 

86.3 

2 

Over  30,000 

56.6 

68.5 

Here  we  see  that  when  less  than  1500  larvae  were  devel- 
oping together,  their  relative  anal  and  oral  arm  lengths 
were  respectively  121.2  and  118.4.  As  the  number  in- 
creased, the  lengths  steadily  dwindled  down,  till  with 
over  30,000  per  litre  they  became  reduced  to  respect- 
ively 56.6  and  68.5,  or  about  half  their  original  amount. 
Now  it  was  found  that  the  body  lengths  of  the  larvae, 

*Phil.  Trans.  1895,  B.  p.  603. 


298  THE  EFFECT  OF  FOOD 

or  the  dimension  measured  in  all  the  observations  on 
larvae  hitherto  described,  was  practically  unaffected  by 
the  "  concentration  "  of  the  larvae.  This  apparent  con- 
tradiction is  easily  accounted  for  by  the  fact  that  the 
times  of  development  of  the  body  and  of  the  arms  of 
the  larvae  is  not  the  same.  At  moderate  temperatures, 
the  body  attains  about  80  per  cent,  of  its  full  length  by 
the  end  of  the  second  day,  and  90  per  cent,  by  the  end 
of  the  third.  The  arms  are  practically  non-existent  at 
the  end  of  the  second  day,  however,  and  attain  only  65 
per  cent,  of  their  full  length  by  the  end  of  the  third. 
As,  therefore,  the  products  of  metabolism  in  the  water 
are  practically  nil  during  the  first  day  or  two,  and  only 
gradually  accumulate  with  progress  of  time,  it  follows 
that  the  growth  of  the  body  tissues  is  unaffected  by 
them,  whilst  that  of  the  arm  tissues  is  restrained. 

The  influence  of  the  excreta  of  adult  Echinoids  upon 
larval  growth  was  then  tested.  Echinoids  of  known 
weight  were  kept  for  a  known  time  in  a  known  volume 
of  water,  so  that,  on  determining  the  absolute  effect 
produced  on  larvae  grown  in  this  water,  it  was  possible 
to  calculate  the  relative  effect  produced  by  unit  weight 
of  Echinoid  kept  for  unit  time  in  unit  volume  of  water. 
On  growing  larvae  in  water  previously  fouled  by  adult 
Echinoids  of  their  own  species,  it  was  found  that,  as  a 
mean  of  five  observations,  they  were  diminished  in  rela- 
tive size  by  2.6  per  cent.,  whilst  only  41  per  cent,  of  the 
ova  employed  reached  the  larval  stage.  On  growing 
them  in  water  fouled  by  Echinoids  of  other  than  their 
own  species,  the  larvae,  as  a  mean  of  five  observations, 
were  diminished  by  only  1.9  per  cent.,  whilst  54  per 
cent,  of  the  ova  reached  the  larval  stage.  That  is  to 


AND  OF  PEODUCTS  OF  METABOLISM.         299 


say,  the  products  of  excretion  of  an  Echinoid  act  more 
adversely  both  on  the  death  rate  and  on  the  growth  of 
embryos  if  these  belong  to  its  own  species,  than  if  they 
belong  to  another  species.  At  least  this  is  the  case 
with  Strongylocentrotus,  Sphcer  echinus,  and  Echinus. 
With  two  other  (physiologically)  less  closely  related 
species,  viz.,  Arbacia  pustulosa  and  Dorocidaris  papil- 
lata,  it  was  even  found  that  the  products  of  excretion, 
so  far  from  acting  adversely  on  growth,  actually  fav- 
oured it.  Thus  Strongylocentrotus  larvae  grown  in 
water  fouled  by  these  two  species  were  increased  in 
size  by  respectively  4.3  and  1.7  per  cent.,  whilst  respect- 
ively 81  and  50  per  cent,  of  the  ova  employed  reached 
the  pluteus  stage. 

It  will  probably  be  thought  that  this  last  result  is 
erroneous;  but  other  observations  showed  that  it  was 
not  so.  Thus  Strongylocentrotus  larvae  were  grown  in 
water  fouled  by  various  other  animals,  and  it  was  found 
that  in  this  case  also  there  was  generally  a  distinct  in- 
crease in  size.  We  see  in  the  accompanying  table  that 


ABSOLUTE 

ABSOLUTE 

AN  I  M  A  LB 
USED  FOR 
POU  LING 

PER   CENT. 
VARIATION 
IN  SIZE  OP 

RELATIVE 
PER  CENT. 
VARIATION 

ANIMALS    USED    FOR 
FOULING  WATER. 

PER   CENT. 
VARIATION 
IN  SIZE  OF 

RELATIVE 
PER     CENT. 
VARIATION. 

WATER. 

LARVAE. 

LARVAE. 

1    Fish 

+  1.4 

+   1.8 

3  Holothurians 

+50 

+2.0 

2   Fish 

+  8.3 

+12.8 

3  Holothurians 

+4.7 

+  -9 

1  Crab  +3  Anem- 

4  Crabs 

+  1.6 

+  2.1 

ones 

—1.5 

—1.0 

30  Molluscs 

+  2.8 

+  1.3 

3  Anemones 

—  .6 

—  .5 

48  Molluscs 

+  4.8 

+  1.1 

1  Medusa 

—2.2 

-1.9 

of  the  ten  observations  made,  a  positive  effect  (averag- 
ing 4.1  per  cent.)  was  produced  in  seven  instances, 


300  THE  EFFECT  OF  FOOD 

whilst  a  much  slighter  negative  effect  (averaging  1.4 
per  cent.)  was  produced  in  only  three.  The  relative 
variation  in  size  produced  by  100  grams  of  animal  foul- 
ing 1  litre  of  water  for  1  hour  is  also  given.  These 
values  are  somewhat  more  variable  than  those  repre- 
senting the  absolute  variation,  but  they  to  some  extent 
corresponded  to  the  amount  of  nitrogenous  matter 
actually  excreted  into  the  water,  as  was  proved  by 
chemical  analysis  of  the  various  samples. 

We  may  conclude,  therefore,  that  under  certain  con- 
ditions products  of  metabolism  may  stimulate  an  organ- 
ism to  increased  growth,  whilst  under  certain  others 
they  may  retard  growth.  What  is  the  nature  of  these 
excretory  products  which  exert  so  potent  an  effect? 
Observations  made  on  the  influence  of  various  simple 
substances  on  larval  growth  seem  to  throw  some  light 
on  the  question.  The  results  obtained  with  uric  acid 
and  urea  are  given  in  the  accompanying  table: 


SUBSTANOB  PRESENT,  AND  AMOUNT. 

PER  CENT.   VARIATION  IN 
SIZE  OF  LARVAE. 

Uric  acid,  1  in  154,000 

+  5.3 

"     "       1  in   70,400 

-j-12.2 

"    "      1  in    58,000 

-f  5.8 

"     "       lin    28,000 

-  2.1 

Urea          lin    65,000 

+  2.3 

lin   59,000 

+  3.7 

lin   44,000 

+  2.2 

Here  we  see  that  uric  acid  in  moderate  amounts 
exerts  a  very  favourable  influence  on  the  size  of  larvae. 
It  is  only  when  the  proportion  is  raised  to  1  in  28,000 
(a  more  than  half  saturated  solution),  that  an  unfav- 


AND  OF  PRODUCTS  OF  METABOLISM.         301 

curable  effect  shows  itself.  Urea  also  acts  favourably, 
though  not  to  the  same  extent  as  uric  acid.  If,  there- 
fore, these  two  simple  bodies  are  capable  of  stimulating 
the  tissues  to  increased  growth,  it  is  possible  that  the 
effects  produced  by  animal  excreta  may  be  due  to 
minute  quantities  of  other  but  more  complex  nitro- 
genous bodies.  That  they  are  not  due  to  simple  urea 
and  uric  acid  was  proved  by  the  chemical  analyses  of  the 
fouled  waters,  for  the  amount  of  nitrogen  found  to  be 
present  was  never  half  sufficient,  and  as  a  rule  was  very 
much  less.  As  to  the  substances  producing  an  ad- 
verse influence  on  growth,  no  definite  evidence  was 
obtained,  but  it  seemed  possible  that  they  might  be 
derivatives  of  ammonia,  perhaps  amines  or  amido- 
bodies.  Thus  ammonium  salts  themselves  exert  an  ex- 
ceedingly poisonous  action,  as  may  be  gathered  from 
the  following  data: 

WEIGHT  OF  AMMONIUM 
CHLOBIDB  PER  LITRE.  EFFECT   PRODUCED. 

.0258  gin.  Larvae  diminished    7.3  per  cent,  in  size. 

.0394  "  "  19.0  per  cent.      " 

.1075  59  per  cent,  blastulse  formed.    Larvae  lived  3 

days. 

.3745  37  per  cent.        "  "  No  larvae. 

.  7890  Most  of  the  ova  had  disintegrated  after  24  hours . 

That  the  effect  produced  by  nitrogenous  bodies  depends 
almost  entirely  upon  the  form  in  which  the  nitrogen  is 
combined,  is  shown  by  the  fact  that  nitrates  and  nitrites 
have  no  influence  on  larval  growth  unless  the  propor- 
tions added  be  over  1  gram  and  .3  gram  per  litre  re- 
spectively. 

The  products  which  every  organism  excretes  prob- 
ably consist,  therefore,  of  various  complex  nitrogenous 


302  THE  EFFECT  OF  FOOD 

bodies,  which  differ  in  different  organisms.  If  they 
come  into  contact  again  with  the  tissues  from  which 
they  have  been  expelled,  they  retard  the  growth  of  these 
tissues,  but  if  with  other  tissues  with  which  they  have 
no  direct  chemical  relation  or  association,  they  may 
under  certain  circumstances  stimulate  them  to  in- 
creased growth. 

The  effects  of  products  of  metabolism  upon  growth 
have  been  tested  at  considerable  length  in  the  case  of 
certain  Molluscs.  At  least  it  is  to  this  influence  that  the 
results  obtained  by  Karl  Semper  and  by  De  Varigny  in 
their  experiments  on  Limncea  stagnalis,  the  common 
pond  snail,  ought,  in  my  opinion,  to  be  ascribed. 
Semper  *  found  that  if  various  numbers  of  the  small 
snails  were  placed  in  equal  volumes  of  water  immedi- 
ately after  hatching,  and  were  kept  there  under  other- 
wise equal  conditions  as  to  food,  temperature,  etc.,  for 
about  two  months,  then  the  size  to  which  they  attained 
was  by  no  means  equal,  but  varied  in  more  or  less  in- 
verse proportion  to  the  number  of  snails  present.  In 
four  very  consistent  experiments,  the  numbers  of  snails 
placed  in  volumes  of  2000  cc.  of  water  were  in  each 
case  respectively  1,  5,  10,  and  20,  or  each  snail  obtained 
respectively  2000,  400,  200,  and  100  cc.  of  water.  The 
lengths  attained  by  the  snails  after  two  months'  growth 
are  given  in  the  table  below. 

Here  we  see  that  snails  allowed  to  grow  singly  in  the 
2000  cc.  vessels  of  water  attained  to  more  than  three 
times  the  size  of  those  grown  in  twenties.  This  was  not 
merely  a  question  of  nutrition,  as  the  amount  of  food 

*  Arb.  a.  d.  Zool.  Inst.  in  Wurzburg,  i.  p.  137, 1874;  also  "  Animal 
Life,"  ed.  4,  p.  51. 


AND  OF  PRODUCTS  OF  METABOLISM. 


303 


supplied  was  always  at  an  optimum.  It  was  evidently 
in  some  way  the  result  of  the  volume  of  water  available 
for  each  snail's  needs.  Other  experiments  in  which  the 
number  of  snails  was  constant,  but  the  volumes  of 
water  unequal,  gave  a  similar  result.  The  manner  in 
which  the  volume  of  water  affected  the  snail's  growth, 
Semper  confessed  himself  unable  to  determine;  but  he 
supposed  that  the  water  must  contain  some  substance, 
as  yet  unknown,  which  is  essential  for  stimulating  the 
growth  of  the  snails.  The  less  of  this  hypothetical 
body  available,  therefore,  the  more  retarded  their 
growth. 


NUMBER    OP 
SNAILS   IN 

VOLUME    OF 
WATER 

LENGTH  IN  MILLIMETRES. 

AVERAGE 

2000  CC. 

PER  SNAIL. 

LENGTH. 

1 

2000  cc. 

17.5 

19.7 

18.5 

17.0 

18.2mm. 

5 

400  cc. 

11.7 

10.1 

10.8 

10.5 

10.8 

10 

200  cc. 

8.8 

7.5 

6.8 

8.6 

7.9 

20 

100  cc. 

6.2 

6.2 

4.6 

5.0 

5.5 

tr' 

Within  recent  years  De  Yarigny  *  has  re-studied  the 
unsolved  problem,  and  has  extended  Semper's  methods 
in  several  directions.  He  used  both  Limn&a  stagnalis 
and  what  he  termed  L.  auricularis,  though  this  form 
was  probably  L.  pereger,  judging  from  his  figures.f 
He  confirmed  Semper's  conclusion  that  the  size  is  in- 
fluenced by  the  number  of  individuals  in  the  vessel,  but 
he  did  not  find  the  snails  nearly  so  sensitive  to  differ- 
ences in  the  volume  of  the  water  as  had  Semper.  Dif- 
ferences in  the  superficial  area  of  the  water  exposed  to 

*  Journ.  de  1'Anat.  et  de  la  Physiol.,  p.  147,  1894. 
fNat.  Sci.  v.  p.  168. 


304  THE  EFFECT  OF  FOOD 

the  atmosphere  he  found  to  be  much  more  important 
than  differences  of  volume.  Thus  a  snail  kept  five 
months  in  a  litre  of  water  having  a  surface  of  18  cm.  in 
diameter  attained  to  nearly  twice  the  length  of  one 
kept  in  an  equal  volume  of  water  which  had  a  surface 
of  only  2  cm.  diameter.  In  order  to  test  Semper's 
hypothesis  of  the  essential  substance  in  the  water,  De 
Yarigny  suspended  a  glass  tube  2  to  3  cm.  in  diameter 
in  various  sized  vessels  of  water.  A  piece  of  muslin 
was  tied  over  the  bottom  of  the  tube,  so  as  to  permit 
of  interchange  of  water,  but  prevent  the  snails  placed 
in  the  tube  and  in  the  outer  vessel  of  water  from  inter- 
migrating.  After  two  to  five  months'  growth  it  was 
found  that  the  snail  placed  in  an  outer  vessel  of  4200 
cc.  capacity  sometimes  attained  to  more  than  twice 
the  length  of  that  placed  in  an  inner  one  of  250  cc. 
capacity.  Again,  snails  were  placed  in  two  tubes  of 
the  same  size,  one  of  which  was  suspended  in  a  vessel 
containing  100  cc.  of  water,  and  the  other  in  a  vessel 
containing  1150  cc.;  in  another  similar  experiment  tl^e 
external  volumes  of  water  were  respectively  50  and  500 
cc.  In  each  case,  however,  the  snails  in  the  two  inner 
vessels  attained  to  practically  the  same  size.  Still 
again,  two  similar  tubes,  holding  50  to  70  cc.  of  water, 
were  placed  in  a  vessel  containing  4200  cc.  of  water. 
One  tube  was  closed  with  muslin,  and  the  other  with  a 
tight-fitting  cork,  which  of  course  prevented  all  inter- 
change with  the  outer  vessel  of  water.  Nevertheless 
the  snail  in  this  tube,  after  two  months'  growth,  was 
only  very  slightly  smaller  than  that  in  the  other 
tube,  but  both  of  them  were  only  about  three-fifths  the 
size  of  the  shell  grown  in  the  external  vessel.  It  should 


AND  OF  PRODUCTS  OF  METABOLISM.         305 

be  mentioned  that  in  all  these  experiments  De  Varigny 
lifted  each  tube  out  of  its  vessel  of  water  and  replaced 
it  two  or  three  times  a  day,  in  order  to  mix  the  water  in 
it  with  that  in  the  external  vessel.  He  concluded, 
therefore,  that  Semper's  hypothesis  is  not  tenable,  and 
that  the  size  of  the  snails  actually  depends  in  some  way 
on  the  volume  of  water  containing  them,  and  on  the 
superficial  area  of  this  water.  His  explanation  of  the 
phenomenon  is  that  in  small  vessels  the  snail  would 
need  to  move  about  less  in  order  to  obtain  food,  for  this 
would  always  be  near  at  hand.  With  less  exercise,  the 
growth  rate  might  accordingly  be  diminished.  This 
explanation  does  not  account  for  some  of  the  principal 
results  obtained  by  Semper  and  by  De  Yarigny  himself, 
however.  Thus  in  vessels  of  equal  volume,  but  con- 
taining various  numbers  of  snails,  the  amount  of  move- 
ment and  exercise  necessary  would  be  just  the  same  in 
each  case,  and  yet,  as  we  have  seen,  the  growth  rate 
varies  enormously. 

In  all  probability,  the  results  obtained  both  by  Sem- 
per and  by  De  Yarigny  can  be  most  simply  accounted 
for  in  the  manner  already  suggested.  Thus  De  Ya- 
rigny actually  found  that  snails  grown  in  water  in  which 
other  snails  had  already  been  growing  several  months 
were  distinctly  smaller  than  those  grown  in  fresh  water, 
and  if  the  excreta  of  snails  had  been  added  as  well,  they 
were  smaller  still.  If,  then,  the  observed  differences 
in  growth  are  due  to  the  accumulation  of  various  quan- 
tities of  products  of  metabolism,  how  can  we  account 
for  the  results  obtained  by  De  Yarigny  in  his  muslin- 
bottomed  tube  experiments?  We  must  imagine  that 
the  mixing  of  the  internal  and  external  waters  two  or 


306  THE  EFFECT  OF  FOOD 

three  times  daily,  and  the  constant  slow  interchange 
through  the  muslin,  were  insufficient  to  equalise  the 
proportions  of  metabolic  products  in  the  two  vessels  for 
more  than  a  short  time,  so  that  on  an  average  the  water 
in  the  inner  vessel  was  more  foul  than  that  in  the 
outer.  This  fouling  would  probably  be  much  increased 
by  particles  of  decomposing  vegetable  matter  and  of 
animal  excreta  collecting  in  the  fibres  of  the  muslin 
and  on  the  inner  walls  of  the  glass  tube,  and  constantly 
poisoning  the  water.  The  outer  water  would  also  be 
fouled  in  this  manner,  but  to  a  very  much  slighter  ex- 
tent, for  the  "  fouling  area  "  of  muslin  and  walls  of  ves- 
sels would  be  proportionately  very  much  less.  That 
the  metabolic  products  from  unhealthy  or  decomposing 
vegetable  matter  can  exert  a  most  harmful  influence  on 
growth  is  shown  by  some  of  my  own  experiments  with 
plutei.  Thus  ova  allowed  to  develop  in  water  which 
had  previously  contained  1  or  2  gm.  per  litre  of  (pre- 
sumably unhealthy)  seaweed,  were  diminished  in  size 
by  as  much  as  13.2  and  18.1  per  cent.* 

De  Varigny's  experiments  on  the  influence  of  super- 
ficial area  of  water  must  be  considered  in  conjunction 
with  some  observations  by  Yung  f  on  tadpoles.  Yung 
put  twenty-five  freshly  hatched  tadpoles  in  each  of 
three  vessels  which  contained  equal  volumes  of  water 
(1200  cc.),  but  of  which  the  diameters  were  respect- 
ively 7  cm.,  11  cm.,  and  14.5  cm.  Thus  the  surface  of 
water  exposed  to  the  air  varied  in  the  proportions  of 
1  :  2.5  :  4.3  After  a  month  and  a  half  the  tadpoles 
were  found  to  have  attained  the  following  average  sizes : 

*  Mittheihmgen  a.  d.  Zool.  Stat.  z.  Neapel.,  Bd.  xiii.  p.  348. 
t  Arch,  des  Sci.  Phys.  et  Nat.,  xiv.  p.  502,  1885. 


AND  OF  PRODUCTS  OF  METABOLISM.        307 


SUPERFICIAL  AREA  OP  WATER. 

1 

2.5 

4.3 

Length  of  tadpoles 
Breadth"        "            ... 
Date  of  first  metamorphosis 

26.2  mm. 
6.1 
Aug.  4 

34.2 

7.8 
July  22 

41.2 
8.8 
June  8 

The  greater  size  of  the  tadpoles  bred  in  the  more  ex- 
posed water  Yung  attributed  to  this  water  absorbing  a 
larger  proportion  of  oxygen  from  the  air.  This  is  in  all 
probability  the  correct  explanation  both  of  these  obser- 
vations and  of  the  similar  ones  of  De  Yarigny  on  snails. 
The  greater  supply  of  oxygen  would  not  only  stimulate 
the  rate  of  growth  of  the  tadpoles  and  of  the  snails,  but 
would  also  hasten  the  oxidation  of  the  harmful  products 
of  metabolism.  It  is  true  that  De  Varigny  found  that 
a  snail  kept  for  eight  months  in  a  corked  vessel  contain- 
ing about  550  cc.  of  water  and  500  cc.  of  air  attained  to 
only  slightly  less  a  size  than  another  snail  kept  in  a 
similar  but  unstoppered  vessel,  but  this  may  have  been 
due  to  the  fact  that  green  plants  were  flourishing 
healthily  in  each  vessel  throughout  the  whole  period, 
and  these  may  have  been  sufficient  to  remove  most  of 
the  metabolic  products  excreted  by  the  snails. 

Further  evidence  as  to  the  influence  of  volume  of 
water  on  the  growth  of  molluscs  has  been  obtained  by 
Whitfield.*  This  observer  kept  a  Limncea  megasoma 
in  a  small  aquarium,  and  after  some  months  it  depos- 
ited eggs.  These  hatched  out,  grew  in  size,  and  in  due 
course  themselves  deposited  eggs.  This  process  con- 
tinued for  four  generations  in  all,  the  shells  of  each 
generation  being  smaller  than  those  of  the  one  before. 

*  Bull.  Amer.  Mus.  Nat.  Hist.,  vol.  i.  p.  29;  and  Amer.  Naturalist, 
xiv.  p.  51. 


308  THE  EFFECT  OF  FOOD 

Those  of  the  last  generation  had  altered  so  much  that  a 
conchologist  of  experience  was  of  the  opinion  that  they 
could  bear  no  possible  specific  relation  to  those  of  the 
first.  Thus  in  addition  to  the  diminution  in  size,  the 
spire  had  become  very  slender.  In  a  second  experi- 
ment of  a  similar  kind,  the  shells  of  the  third  genera- 
tion were  only  4-7ths  as  long  as  those  of  the  parent 
stock,  and,  still  more  remarkable,  the  male  organs  had 
disappeared,  whilst  the  liver  had  become  considerably 
reduced  in  size. 

These  extraordinary  effects  were  probably  due  to  the 
cumulative  action  of  the  increasing  quantities  of  meta- 
bolic products  in  the  water  in  which  the  molluscs  were 
living. 

Still  another  series  of  observations  on  the  effect  of  a 
confined  volume  of  water  was  described  by  Warren  * 
only  a  year  or  two  ago.  These  were  made  upon 
Daphnia  magna  (Water-flea).  Four  adult  individuals 
were  placed  separately  in  covered  glass  vessels  contain- 
ing 200  cc.  of  water,  together  with  some  Conferva  and 
some  mud  containing  algSB,  etc.  Four  others  were 
placed  in  similar,  but  uncovered  vessels,  and  four  more 
in  still  other  vessels,  of  which  the  water  was  changed 
about  once  a  day.  The  water  in  the  former  vessels  was 
never  changed,  but  the  loss  due  to  evaporation  in  the 
uncovered  vessels  was  compensated  for  by  the  occasional 
addition  of  rain  water.  The  Daphnias  produced  broods 
of  four  or  five  offspring  each  after  about  15  days,  and 
these  offspring  were  allowed  to  grow  in  the  vessels,  and 
after  a  time  produced  offspring  in  their  turn.  It  was 
interesting  to  note,  however,  that  in  the  vessels  in 
*Q.  J.  Microsc.  Sci.,  vol.  43,  p.  212,  1900. 


AND  OF  PRODUCTS  OF  METABOLISM.    309 

which  the  water  remained  unchanged,  the  rate  of  repro- 
duction and  the  number  of  offspring  in  a  brood  was  con- 
siderably diminished.  The  third  generation  was  pro- 
duced about  22  days  after  the  second,  and  the  fourth 
about  25  days  after  that,  and  then  the  breeding 
stopped.  In  the  vessels  with  frequently  changed 
water,  the  third  to  seventh  generations  were  produced 
at  intervals  of  respectively  18,  14,  15,  16,  and  26 
days,  and  then  for  some  unknown  reason  the  families 
died  out.  The  confined  volume  of  water  had  another 
and  even  more  remarkable  effect,  however,  as  it 
caused  a  distinct  shortening  in  the  length  of  the  spine 
formed  by  the  posterior  prolongation  of  the  cara- 
pace. Thus  in  one  series  of  observations  it  was  reduced 
from  a  length  of  241  (relative  to  the  carapace  length 
taken  as  1000)  in  the  parents  to  one  of  171  in  the  off- 
spring; and  in  another  series  from  a  length  of  276  in 
the  parents  to  one  of  249  in  the  children,  and  185  in  the 
grandchildren.  In  this  latter  case,  therefore,  it  would 
seem  as  if  the  acquired  character  of  shortened  spine 
was  inherited. 

Warren  attributes  these  remarkable  effects  to  the  ex- 
cretory matter  thrown  off  by  the  Daphnias  into  the 
water.  Also  he  concluded  that  this  matter  "  may 
feasibly  be  supposed  to  be  particularly  injurious  to 
Daphnia;  for  when  the  Daphnia  are  fast  disappearing, 
there  may  be  a  swarm  of  Ostracods  or  Copepods  (still 
living  healthily  in  the  water)."  In  fact  Warren  in- 
clined to  the  view,  already  suggested  by  the  author  in 
the  case  of  Echinoids  and  other  marine  animals,  that 
water  fouled  by  Daphnia  "  becomes  specifically  injuri- 
ous to  Daphnia." 


CHAPTEE  X. 

THE    EFFECTS    OF    CONDITIONS    OF    LIFE    IN 
GENERAL. 

Local  conditions  of  life  perhaps  the  cause  of  local  races,  but  proof 
of  this  is  as  a  rule  impossible — American  and  European  trees 
compared — Alpine  and  Arctic  plants — Effects  of  cultivation — 
Local  races  of  oysters  and  of  snails — Lepidoptera  in  Malay  Archi- 
pelago— Local  races  of  shrimps,  of  mackerel,  and  of  herring — 
North  American  birds  and  mammals— Action  of  climate  on  goats 
and  on  rabbits— Effect  of  domestication  on  rabbits,  pigeons,  fowls, 
and  ducks. 

IN  the  three  preceding  chapters  we  have  examined 
numerous  cases  of  variation  produced  wholly  or  in  great 
part  by  a  change  in  some  one  condition  of  environment. 
In  the  present  chapter  no  such  attempt  is  made  to  trace 
an  effect  to  any  single  cause,  but  we  shall  examine  the 
effects  of  all  conditions  of  life  together,  such  as  climate, 
nutrition,  moisture,  and  sunlight,  in  the  production  oi 
variations.  The  variations  more  particularly  to  be 
studied  are  those  which  are  common  to  whole  groups 
of  organisms,  and  which  go  to  form  what  are  known  as 
local  races.  Unfortunately  in  the  majority  of  cases  it 
is  impossible  to  prove  that  such  races  are  the  direct  or 
indirect  results  of  the  differences  of  environment,  even 
when  there  is  a  high  probability  that  such  is  the 
case.  Hence,  when  local  races  are  referred  to,  it  is  not 
intended  to  imply  that  the  distinguishing  characters 
exhibited  are  definitely  due  to  the  action  of  the  environ- 

310 


THE  EFFECTS  OF  CONDITIONS  OF  LIFE.     311 

ment.  The  inference  is  only  that  they  may  be,  if  not 
wholly,  then  in  part. 

Among  plants,  a  striking  instance  of  the  apparently 
direct  action  of  conditions  of  life  in  the  production  of 
variations  has  been  afforded  by  Meehan.*  This  ob- 
server "  has  compared  twenty-nine  species  of  American 
trees  with  their  nearest  European  allies,  all  grown  in 
close  proximity  and  under  as  nearly  as  possible  the  same 
conditions.  In  the  American  species  he  finds,  with  the 
rarest  exceptions,  that  the  leaves  fall  earlier  in  the  sea- 
son, and  assume  before  they  fall  a  brighter  tint;  that 
they  are  less  deeply  toothed  or  serrated;  that  the  buds 
are  smaller;  that  the  trees  are  more  diffuse  in  growth 
and  have  fewer  branchlets;  and  lastly,  that  the  seeds 
are  smaller — all  in  comparison  with  the  corresponding 
European  species."  f  The  trees  compared  belong  to 
several  distinct  orders,  and  are  adapted  to  widely  dif- 
ferent stations,  hence  Darwin  considers  that  the  ob- 
served differences  should  be  "  attributed  to  the  long 
continued  action  of  a  different  climate." 

More  conclusive  evidence  of  the  direct  effect  of  en- 
vironment has  been  obtained  in  the  case  of  Alpine 
plants.  The  especially  characteristic  features  of  these 
plants,  as  compared  with  similar  or  allied  plants  growing 
at  lower  levels,  are  a  dwarfing  in  size  and  compactness 
of  growth  sometimes  giving  rise  to  a  moss-like  appear- 
ance; a  more  intense  green  colour  in  the  leaves,  and 
greater  brilliancy  and  size  in  the  flowers;  an  increased 
hairiness  of  the  leaves,  and  occasionally  a  certain  degree 
of  fleshiness  of  the  tissues.  Now  by  growing  lowland 

*  Proc.  Acad.  Nat.  Sci.  of  Philadelphia,  January  28,  1862. 
f  Quoted  from  "Animals  and  Plants,"  ii.  p.  271. 


312  THE  EFFECTS  OF  CONDITIONS 

plants  at  high  altitudes,  Bonnier,*  Flahault,t  and  others 
have  shown  that  such  characters  as  these  may  be  rapidly 
acquired.  For  instance,  Bonnier  made  observations  on 
Teucrium  Scorodonia  for  no  less  than  eight  years,  and 
he  found  that  this  plant,  when  sown  at  a  high  situation 
in  the  Pyrenees,  produced  very  short  aerial  stems,  with 
more  hairy  and  darker  green  leaves,  and  more  compact 
inflorescence.  On  the  other  hand,  seeds  gathered  from 
plants  growing  at  high  altitudes,  and  sown  in  Paris, 
after  three  years  produced  elongated  stems,  with  less 
hairy  and  brighter  green  leaves,  or  plants  very  similar 
to  those  from  seeds  obtained  in  the  neighbourhood  of 
Paris.  The  modifications  acquired  during  a  given  time 
by  a  lowland  plant  grown  at  a  high  level,  or  a  highland 
plant  grown  at  a  low  level,  took  about  the  same  amount 
of  time  to  disappear,  on  returning  the  plants  to  their 
primitive  climates.  Again,  Bonnier  found  that  plants 
of  Lotus  corniculatus  from  Alpine  situations  had  a  very- 
thick  epidermis,  a  collenchymatous  cortex,  and  a  rela- 
tive reduction  of  the  wood.  Those  cultivated  in  lower 
altitudes  had,  on  the  other  hand,  a  thinner  epidermis, 
a  cortex  without  collenchyma,  and  an  increased  devel- 
opment of  wood. 

With  reference  to  the  inflorescence,  "  Dr.  Schubeler 
sowed  seeds  of  various  plants  in  different  latitudes  in 
Norway,  and  proved  that  the  brilliancy  of  the  flowers 
increased  with  the  latitude.  So  great  were  the  differ- 
ences that  it  was  difficult  to  conceive  that  they  were 
produced  from  the  same  batch  of  seeds.  The  differ- 

»  Ann.  Sci.  Nat.  Bot.,  vii  serie,  xx.  p.  217,  1894. 
t  Ann.  Sci.  Nat.  Bot.,  p.  159,  1879,  and  Rev.  Gen.  de  Bot.,  ii. 
p.  513,  1891. 


OF  LIFE  IN  GENERAL.  313 

ences  appeared  in  the  first  year.  Similarly,  seeds 
from  Germany  exhibited  analogous  differences."  * 
Dr.  Schubeler  also  observed  an  increased  greenness  of 
the  foliage. 

The  Arctic  climate,  though  similar  in  many  respects 
to  the  Alpine,  yet  differs  considerably  in  others.  By 
comparing  plants  from  the  Islands  of  Spitzbergen  and 
Jan  Mayen,  with  specimens  of  the  same  species  col- 
lected in  the  Alps  and  the  Pyrenees,  Bonnier  f  has 
shown  that  there  are  modifications  of  structure  cor- 
responding to  these  differences  of  environment.  The 
Arctic  plants  have  more  rounded  cells  and  more  con- 
siderable intercellular  spaces  in  their  tissues,  whilst  the 
cuticle  is  diminished  in  thickness,  and  the  vessels  are 
diminished  in  number  and  in  calibre.  These  changes 
towards  an  incipiently  aquatic  type  are  probably  due  to 
the  greater  humidity  of  the  air.  The  fleshiness  of  the 
leaves  Bonnier  attributes  to  the  continuous  solar  illumi- 
nation, though  it  may  perhaps  be  due  to  the  neighbour- 
hood of  the  plants  to  the  sea. 

The  effect  of  cultivation  on  the  variation  of  plants  is 
well  known  to  be  in  many  cases  exceedingly  great;  but 
in  hardly  any  of  the  recorded  cases  is  any  mention  made 
of  the  extent  to  which  artificial  selection  was  practiced. 
One  cannot  tell,  therefore,  how  much  ought  to  be  at- 
tributed to  the  direct  action  of  the  environment,  and 
how  much  to  selection.  The  following  instance,  how- 
ever, seems  to  be  the  direct  result  of  cultivation. 
It  concerns  the  spiderwort,  Tradescantia  virginica. 

*  Quoted    from    Henslow's    "Origin    of   Plant    Structures,"  p. 
118. 
fRev.  Gen.  Bot.,  vi.  p.  505,  1894. 


314  THE  EFFECTS  OF  CONDITIONS 

G.  A.  Brennan  *  records  that  he  set  out  this  plant  in 
1872,  giving  it  very  rich  treatment.  "  In  1874  it  be- 
gan to  deviate  from  the  original  trimerous  type  and  to 
assume  the  tetramerous  one,  by  developing  another 
petal,  and  instead  of  doing  this  at  the  expense  of  the 
pistil  or  stamens,  it  added  another  sepal,  another  carpel 
with  style,  and  two  stamens,  thus  making  a  typically 
tetramerous  flower."  In  1876  a  pentamerous  plant  was 
evolved;  in  1879  a  hexamerous;  in  1882  a  dimerous; 
and  in  1884  a  heptamerous.  Thus  as  the  result  of  thir- 
teen years  of  cultivation,  "a  monocotyledonous  plant 
has  in  bloom,  at  the  same  time,  flowers  of  dimerous, 
trimerous,  tetramerous,  pentamerous,  hexamerous,  and 
heptamerous  types  respectively,  each  flower  having 
twice  as  many  stamens  as  sepals,  petals,  or  carpels  of 
ovary." 

To  turn  to  the  Animal  Kingdom,  an  interesting  in- 
stance of  variation  following  directly  on  change  of  en- 
vironment is  that  noticed  by  Costa  f  in  the  oyster. 
Thus,  on  transferring  young  oysters  from  English  shores 
to  the  Mediterranean,  it  was  found  that  their  manner 
of  growth  at  once  altered,  and  prominent  diverging  rays 
were  formed,  like  those  on  the  shells  of  the  native 
Mediterranean  oyster.  The  variations  noticed  by  Ley- 
dig  $  in  the  snail  Helix  nemoralis  are  attributed  by  him 
to  the  direct  influence  of  environment,  and  this  may  be 
actually  the  case,  but  there  is  no  evidence  to  prove  it. 
He  noticed  that  at  Mainz  the  shell  of  this  snail  exhibits  a 
fine  citron  yellow.  This  hue  disappears  further  down  the 

*Amer.  Naturalist,  vol.  xx.  551,  1886. 

f  Quoted  from  "  Animals  and  Plants,"  ii.  p.  270. 

J  Eimer's  "  Organic  Evolution,"  p.  137. 


OF  LIFE  IN  GENERAL.  315 

Rhine,  and  at  Bonn  and  in  the  still  lower  reaches  the 
red  of  the  snail  deepens  to  a  chocolate  brown.  Cock- 
erell*  also  has  noticed  how  sensitive  is  this  species  of 
snail  to  a  changed  environment.  Thus  it  was  intro- 
duced from  Europe  into  Lexington,  Virginia,  a  few 
years  ago,  and  has  since  then  varied  extraordinarily. 
Already,  in  1890,  125  varieties  had  been  discovered  in 
this  locality.  Of  these  no  less  than  67  were  new,  and 
unknown  in  Europe.  The  variations  noticed  by  Gu- 
lick t  in  the  land  snails  of  the  Sandwich  Islands  may 
also  be  due  partly  to  the  effects  of  environment.  In  a 
small  forest  region  about  forty  miles  by  six  miles  in 
area,  in  the  Island  of  Oahu,  Gulick  found  about  175 
different  species,  represented  by  700  or  800  varieties. 
Successive  valleys  often  showed  allied  species  belong- 
ing to  the  same  genus,  and  Gulick  noticed  that  in  every 
case,  "  the  valleys  that  are  nearest  to  each  other  fur- 
nish the  most  nearly  allied  forms;  and  a  full  set  of  the 
varieties  of  each  species  presents  a  minute  gradation  of 
forms  between  the  more  divergent  types  found  in  the 
more  widely  separated  localities."  Only  a  very  few  of 
the  species  ranged  over  the  whole  Island,  most  of  them 
extending  over  only  five  or  six  miles,  and  a  few  over 
only  one  or  two  square  miles.  These  variations  did  not 
appear  to  be  due  to  the  action  of  the  environment,  as 
the  food,  climate,  and  enemies  in  the  different  valleys 
seemed  to  be  the  same.  Also  the  snails  on  the  rainy 
side  of  the  mountains  did  not  differ  any  more  from 
those  on  the  dry  side  than  they  did  from  those  inhabit- 
ing a  neighbouring  wet  valley  an  equal  distance  away. 

*  Nature,  vol.  li.  p.  393. 

f  Journ.  Linn.  Soc.  (Zool).,  vol.  xi.  p.  496. 


316  THE  EFFECTS  OF  CONDITIONS 

As  Wallace  points  out,*  however,  "  it  is  an  error  to 
assume  that  what  seem  to  us  identical  conditions  are 
really  identical  to  such  small  and  delicate  organisms  as 
these  land  molluscs." 

Upon  Lepidoptera,  we  have  seen  in  a  previous  chap- 
ter that  the  effect  of  particular  conditions  of  environ- 
ment, such  as  temperature  and  nutrition,  may  be 
considerable.  One  would  imagine,  therefore,  that 
changes  in  the  conditions  of  life  as  a  whole  might  form 
an  even  more  potent  source  of  variation.  Conclusive 
evidence  upon  this  point  is,  unfortunately,  almost  un- 
obtainable, though  of  the  inconclusive  kind  which  forms 
the  larger  part  of  this  chapter  there  is  plenty.  For  in- 
stance, Wallace  f  came  to  the  conclusion  that,  with 
reference  to  the  local  forms  occurring  in  the  Indian  and 
Malayan  regions,  "  larger  or  smaller  districts,  or  even 
single  islands  give  a  special  character  to  the  majority 
of  their  Papilionidse.  For  instance :  The  species  of  the 
Indian  region  (Sumatra,  Java,  and  Borneo)  are  almost 
invariably  smaller  than  the  allied  species  inhabiting 
Celebes  and  the  Moluccas.  The  species  of  New  Guinea 
and  Australia  are  also,  though  in  a  less  degree,  smaller 
than  the  nearest  species  or  varieties  of  the  Moluc- 
cas. .  .  The  species  and  varieties  of  Celebes  possess  a 
striking  character  in  the  form  of  the  anterior  wings, 
different  from  that  of  the  allied  species  and  varieties  of 
all  the  surrounding  islands.  Tailed  species  of  India  or 
the  Indian  region  become  tailless  as  they  spread  east- 
ward through  the  Archipelago;  in  Amboyna  and  Ceram 
the  females  of  several  species  are  dull-coloured,  while 

*"  Darwinism,"  p.  148. 

t"  Contributions  to  Natural  Selection,"  p.  167,  1870. 


OF  LIFE  IN  GENERAL.  317 

in  the  adjacent  islands  they  are  more  brilliant."  By 
actual  measurement,  Wallace  found  that  "  no  less  than 
fourteen  Papilionidse  inhabiting  Celebes  and  the  Moluc- 
cas are  from  one-third  to  one-half  greater  in  extent  of 
wing  than  the  allied  species  representing  them  in  Java, 
Sumatra,  and  Borneo.  Six  species  inhabiting  Amboyna 
are  larger  than  the  closely  allied  forms  of  the  northern 
Moluccas  and  New  Guinea  by  about  pne-sixth.  These 
include  almost  every  case  in  which  closely  allied  species 
can  be  compared."  There  are  equally  distinct  local 
variations  of  form  and  colour.  For  instance,  almost 
every  Papilio  inhabiting  Celebes  has  wings  of  a  pe- 
culiar shape,  which  distinguish  it  from  the  allied  species 
of  every  other  island.  Thus  the  upper  wings  are  more 
elongate  and  falcate,  and  the  anterior  margin  is  much 
more  curved. 

A  remarkable  instance  of  the  direct  effects  of  food, 
or  perhaps  of  conditions  of  life  in  general,  is  mentioned 
by  Darwin,  who  was  himself  informed  of  it  by  Moritz 
Wagner.  "  A  number  of  pupae  were  brought  in  1870 
to  Switzerland  from  Texas  of  a  species  of  Saturnia 
widely  different  from  European  species.  In  May,  1871, 
the  moths  developed  out  of  the  cocoons,  and  resembled 
entirely  the  Texan  species.  Their  young  were  fed  on 
leaves  of  Juglans  regia  (the  Texan  form  feeding  on 
Juglans  nigra),  and  they  changed  into  moths  so  differ- 
ent, not  only  in  colour,  but  also  in  form,  from  their 
parents,  that  they  were  reckoned  by  entomologists  as  a 
distinct  species."  *  -* 

Reference  has  already  been  made  in  a  previous  chap- 

*  Quoted  from  Beddard's  "  Animal  Colouration,"  p.  51. 


318 


THE  EFFECTS  OF  CONDITIONS 


ter  to  the  observations  made  by  Weldon  *  on  the  corre- 
lation between  certain  dimensions  in  local  races  of  the 
shrimp.  The  degree  of  correlation  was  found  by  him 
to  be  practically  constant,  but  the  mean  measurements 
themselves  show  distinct  differences  in  the  various  local 
races.  The  variability  or  range  of  variation  of  the 
measurements  about  their  mean  shows  much  greater 


LOCALITY. 

lit 

.« 

!„§,. 

M  O 

MSB 

a    0    H* 

il 

ii|| 

1000  shrimps 
800 

from  Plymouth 
"      Southport 

249.63 
248.31 

4.55 
3.96 

177.53 
180.29 

3.50 
3.65 

500 

Roscoff 

251.51 

3.32 

178.00 

3.02 

380 

"      Sheerness 

247.33 

3.29 

179.68 

2.91 

300 

14      Helder 

251.38 

4.36 

181.67 

4.02 

differences  still.  As  we  see  in  the  accompanying 
table,  the  total  carapace  length  (expressed  in  terms 
of  the  body  length,  taken  as  1000)  varies  in  different 
localities  from  247.33  to  251.38,  or  by  1.6  per  cent. 
However,  the  probable  error  of  variation  of  this  dimen- 
sion varies  from  3.29  to  4.55,  or  by  no  less  than  38.3 
per  cent.  The  mean  post-spinous  carapace  length 
varies  in  its  extreme  limits  by  2.06  per  cent.,  and  its 
probable  error  by  38.1  per  cent.  Now  in  that  the 
shrimps  differ  so  little  in  their  average  dimensions,  they 
cannot  be  very  divergent  races,  and  hence  one  must 
conclude — supposing,  of  course,  that  the  samples  meas- 
ured were  fair  ones,  collected  under  similar  conditions 
— that  the  differences  in  the  variability  of  the  shrimps 
obtained  from  the  various  regions  are  due  chiefly  to  the 
*Proc.  Roy.  Soc.,  vol.  li.  p  2,  1892. 


OF  LIFE  IN  GENERAL. 


319 


action  of  a  more  or  less  correspondingly  variable  en- 
vironment. It  follows,  therefore,  that  the  environ- 
ment at  Plymouth  and  at  Helder  is  more  variable  than 
that  at  Roscoff  and  at  Sheerness. 

To  turn  from  marine  Invertebrates  to  marine  Verte- 
brates, the  local  races  of  the  Mackerel  have  recently 
been  studied  in  considerable  detail  by  Garstang.* 
Some  of  the  chief  of  the  important  results  obtained  by 
him  are  embodied  in  the  accompanying  table : 


MEAN    NUMBER   OF 

LOCALITY 

Sde 

ill 

1 

1? 

a 

S.9  ^ 

iis 

«  w  s 

§    " 

S 

PH" 

P^  c3  j£ 

3  \S''*7t' 

»o« 

gaa 

S3  2 

&jq 

II 

c£a 

SSB 

lill 

£p^  S 

(  Lowestoft 
-<  Ramsgate 
f  Plymouth 

300 
100 
300 

26.75 
26.88 
26.79 

18 
28 
20 

1-12.14 
12.00 

j-84.3 
81.3 

[94.5 

92.3 

J  Scilly 
1  Brest 

74 
100 

26.82 
26.85 

18 
26 

j-  12.16 

j-82.2 

j-93.0 

j  Kinsale 

410 

27.15 

19 

12.14 

85  4 

94.4 

1  Kerry 
Newport,  U.  S.  A. 

245 
100 

27.27 
27.38 

10 

66 

12.33 
11.88 

85.3 
63.0 

94.3 
97.0 

The  number  of  black  transverse  bars  or  stripes  across 
the  sides  of  the  fish  was  found  to  vary  from  23  to  33, 
27  being  in  almost  every  instance  the  most  frequently 
occurring  number.  The  differences  in  the  numbers  of 
bars  occurring  in  the  various  samples  do  not  seem  very 
great,  but  it  is  noticeable  that  all  the  samples  from  the 
North  Sea  and  English  Channel  had  invariably  less 
than  27  bars  (on  an  average),  whilst  those  from  the 
coasts  of  Ireland  and  America  had  more  than  27.  On 


Journ.  Marine  Biol.  Soc.,  vol.  v.  p.  235,  1898. 


320  THE  EFFECTS  OF  CONDITIONS 

classifying  the  fish  according  to  the  number  of  bars,  it 
was  found  that,  of  those  in  the  English  Channel  and 
North  Sea  (including  Brest  and  Scilly),  20  to  22  per 
cent,  had  28  or  more  bars;  of  those  on  the  Irish  coasts, 
34  to  38  per  cent.;  and  of  those  on  the  American  coast, 
no  less  than  44  per  cent.  The  proportions  of  fish  hav- 
ing one  or  more  round  black  dorso-lateral  intermediate 
spots  situated  between  the  transverse  bars,  showed 
even  more  distinct  differences.  Thus  21  per  cent,  of 
the  fish  from  the  North  Sea  and  English  Channel  were 
spotty;  22  percent,  of  those  from  Brest  and  Scilly;  only 
15  per  cent,  of  those  from  Ireland;  but  no  less  than  66 
per  cent,  of  those  from  America. 

It  was  found  that  the  number  of  fin-rays  in  the  first 
dorsal  fin  varied  somewhat  according  to  the  size  of  the 
fish,  it  being,  for  instance,  12.33  in  Irish  fish  under  13 
inches  long,  and  11.92  in  those  of  15  or  more  inches. 
To  get  rid  of  this  variable  factor,  only  fish  13  inches 
long  were  compared.  Here  again  the  American  fish 
showed  the  greatest  divergence  from  the  general  mean, 
whilst  the  Brest  and  Scilly  fish  were  more  or  less  mid- 
way between  the  North  Sea  and  Channel  fish  on  the 
one  hand,  and  the  Irish  on  the  other.  As  regards  the 
second  dorsal  fin,  the  variation  in  the  number  of  fin- 
rays  is  much  slighter  than  for  the  first  dorsal  fin,  it 
being  practically  only  from  11  to  13  (as  against  10  to 
15).  The  American  fish  showed  a  much  wider  varia- 
tion than  any  of  the  others,  only  63  per  cent,  of  them 
having  the  modal  number  of  12  fin-rays,  whilst  the  two 
samples  of  Irish  fish  showed  least  variation,  85.3  per 
cent,  and  85.4  per  cent,  of  them  respectively  having  12 
fin-rays.  In  the  number  of  dorsal  finlets  the  American 


OF  LIFE  IN  GENERAL.  321 

fish  again  showed  the  widest  variation,  only  79  per 
cent,  of  them  having  5  finlets.  The  Irish  fish  again 
showed  the  smallest  variation,  94.4  per  cent,  of  them 
having  5  finlets,  as  against  93.6  per  cent,  in  North  Sea 
and  Plymouth  samples,  and  93.0  per  cent,  in  those  from 
Brest  and  Scilly. 

It  is  obvious,  therefore,  that  the  American  mackerel 
constitute  a  distinct  variety  or  race,  the  most  notice- 
able characteristic  of  which  is  the  high  degree  of  spot- 
tiness.  Garstang  is  of  the  opinion,  also,  that  the 
mackerel  which  frequent  the  British  coasts  should  be 
subdivided  into  two  principal  races:  an  Irish  race,  and 
an  English  Channel  and  North  Sea  race.  The  chief 
differences  between  these  two  subdivisions  lie  in  the 
number  of  transverse  bars  and  of  spots,  and  to  a  lesser 
degree,  of  dorsal  fin-rays  and  finlets.  It  is  a  striking 
fact,  also,  that  "  these  peculiarities  are  greatest  between 
the  races  of  localities  which  are  geographically  remote, 
and  least  between  those  which  occupy  areas  that  are 
geographically  contiguous.  Between  the  mackerel  of 
the  North  Sea  and  English  Channel  there  are  no  dif- 
ferences at  all;  but  the  Irish  race  is  distinctly  divisible 
into  two  stocks,  one  of  which  is  restricted  to  the  west 
coast,  and  the  other  to  the  south." 

Into  the  causes  of  the  variations  shown  by  these  local 
races  Garstang  does  not  enter.  It  is  highly  improba- 
ble that  all  of  the  observed  differences  are  the  direct 
or  indirect  result  of  differences  of  environment,  but  it 
is  possible  that  some  of  them,  such  as  the  bars  and 
spots,  and  size  of  the  fish  (and  with_  this  the  number 
of  dorsal  fin-rays),  may  be  considerably  influenced 
thereby. 


322  THE  EFFECTS  OF  CONDITIONS 

The  local  races  of  the  herring  have  been  studied  by 
Dr.  Friedrich  Heincke  *  with  even  greater  minuteness 
than  those  of  the  mackerel  by  Garstang.  Samples  of 
herring  from  no  less  than  a  hundred  different  locali- 
ties were  examined,  most  of  them  in  respect  of  about 
25  different  characters,  and  some  in  respect  of  over  50 
characters.  Heincke  came  to  the  conclusion  that  the 
various  local  races  of  herring  examined  by  him  differed 
from  each  other  in  the  very  characters  which  are  used 
to  differentiate  the  species  of  the  genus  Clupea  from 
each  other,  though,  as  a  rule,  the  differences  were 
smaller.  For  instance,  the  most  extreme  variations 
noticed  in  the  average  number  of  vertebrae  ranged  from 
57.6  in  the  Norwegian  spring  herring  to  53.6  in  the 
White  Sea  herring,  or  a  difference  of  four  vertebrae. 
The  average  number  of  vertebrae  in  the  species  Her- 
ring can  be  taken  as  56,  or  eight  more  than  in  the 
Sprat,  which  can  be  taken  as  having  48.  On  the  other 
hand  the  species  Clupea  pilchardus  has,  on  an  average, 
about  52  vertebrae,  or  does  not  differ  any  more  from 
the  sprat  on  the  one  hand,  and  the  herring  on  the  other, 
than  do  the  most  widely  divergent  local  races  of  the 
herring. 

Heincke  found  that,  as  a  rule,  the  more  widely  the 
races  are  separated  from  each  other  geographically,  or 
rather,  the  more  their  environmental  conditions  differ, 
the  more  do  they  differ  in  respect  of  certain  characters. 
For  instance,  the  number  of  vertebrae,  and  of  scales 
between  the  ventral  fin  and  the  anus,  showed  the  fol- 
lowing mean  variations: 

*  "  Naturgeschichte  des  Herings,"  Abhandl.  d.  Deutsch.  Seefisch- 
erei-Vereins,  Bd.  ii.  Heft  i.  u,  ii. 


OF  LIFE  IN  GENERAL. 


323 


LOCAL    HACK. 

VERTKBIUE. 

SCALES. 

sP( 

Au 

ring  her 

umu 

ring,  Norway, 
Schley, 
Great  Belt,  . 
Rllgen, 

57.6 
55.5 
55.8 
56.0 
53.6 
55.3 
56.5 
56.4 
56.6 
55.7 

14.0 
13.7 
14.4 
13.9 
12.4 
14.3 
14.8 
15.0 
14.5 
14.5 

White  Sea,  . 
Zuidersee,    . 
E.  Coast,  Scotlan 
North  Sea  (S.  E.) 
Jutland  Bank, 
Baltic  Sea  (W.), 

i, 

Heincke  seems  to  be  of  the  opinion  that  these  differ- 
ences are  largely  the  direct  result  of  the  environment, 
for  he  says  that  all  the  local  races  of  herring  are  sub- 
jected to  a  very  complex  combination  of  environmental 
conditions,  and  that  these  react  upon  them  to  produce 
their  especial  characters.  The  White  Sea  herring  is 
the  most  divergent  from  the  general  mean  in  respect 
of  other  characters  besides  the  number  of  vertebra)  and 
of  scales.  Thus  it  has  only  2  to  10,  or,  on  an  average, 
6  vomerine  teeth,  mostly  in  a  single  series,  whilst  other 
races  have,  on  an  average,  10  to  20  teeth,  arranged  in 
several  series. 

Heincke  finds  that  the  spring  herring  and  the  au- 
tumn herring  are  two  more  or  less  distinct  races,  not 
only  in  the  Baltic  Sea,  but  in  other  localities  as  well 
(West  and  East  coasts  of  Scotland,  North  Sea,  etc.). 
The  spring  herring  differs  from  the  autumn  herring  in 
that  it  is,  as  a  rule,  considerably  larger;  it  has  longer 
anal  fins,  and  often  a  larger  number  of  vertebrae.  It 
always  has  a  smaller  number  of  keeled  ventral  scales, 
and  a  narrower  skull,  and  very  seldom  has  less  than  9 
rays  in  its  ventral  fins.  Autumn  herring  with  8  ven- 
tral fin-rays  occur  fairly  frequently,  however  (20  to  30 


824  THE  EFFECTS  OF  CONDITIONS 

per  cent,  of  all  individuals).  These  differences  of 
character  Heincke  attributes  very  largely  to  the  differ- 
ent conditions  of  development.  Thus,  as  regards  the 
Western  Baltic  herring,  the  larvae  of  the  spring  brood, 
developing  in  the  warm  and  less  saline  waters  of  the 
Schley,  reach  the  young  herring  stage  within  three  or 
four  months.  Those  of  the  autumn  brood,  on  the  other 
hand,  which  hatch  in  the  more  saline  waters  of  the 
open  sea,  need  the  whole  winter  and  spring,  or  7  or  8 
months,  to  reach  the  same  stage. 

The  fish  of  the  Atlantic  and  Pacific  slopes  have  been 
studied  and  compared  by  Eigenmann.*  In  the  eight 
families  compared,  the  number  of  species  on  the  At- 
lantic slope  was  more  than  twice  as  great  as  on  the 
Pacific,  but,  nevertheless,  the  variation  in  the  number 
of  fin-rays  in  the  Pacific  species  was  greater  in  all  but 
two  of  the  families.  The  author  considers  that  this 
may  be  due  to  the  fauna  being  of  diverse  origin,  and  to 
its  being  comparatively  new,  and  not  yet  in  a  state  of 
equilibrium.  The  fish  Leuciscus  balteatus  was  studied 
in  detail,  and  it  was  found  that  every  locality  in  which 
it  was  examined  had  a  variety  peculiar  to  itself.  As  a 
rule,  the  lower  the  elevation  of  the  locality  from  which 
the  fish  were  obtained,  the  greater  the  number  of  fin- 
rays.  The  following  are  the  mean  values  in  support 
of  this  statement: 

NUMBER  OF         AVERAGE 

ELEVATION.  SPECIMENS  EX-      NUMBER  OF 

AMINED.  RATS. 

1—750  feet,                189  18.4 

1078— 2000  feet,  .        .        .        .        .  234  16.6 

2001— 3100  feet, 388  17.5 

5000  feet  or  more, 10  16.0 

*  Amer.  Naturalist,  xxix.  p.  10,  1895. 


OF  LIFE  IN  GENERAL. 


325 


From  his  extensive  researches  on  the  variation  and 
distribution  of  mammals  and  birds  in  North  America, 
J.  A.  Allen  *  has  been  able  to  arrive  at  several  general- 
ised conclusions  concerning  the  geographical  distribu- 
tion of  local  races.  Thus  he  finds  that,  as  a  rule,  the 
mammals  and  birds  of  North  America  increase  in  size 
as  we  pass  from  the  southern  towards  the  northern 
regions.  In  the  accompanying  table  are  given  the 
mean  values  obtained  by  him  for  the  length  of  body, 


PER  CENT.  DIFFERENCE  IN 

DIMENSIONS  OP  SOUTH- 

CORRESPONDING DIMEN- 

ERN SPECIMENS. 

SIONS     OF     NORTHERN 

SPECIMENS. 

SPECIES. 

X 

w 

CO 

£3 

«• 

A 

^J 

s 

I 

|I 

1 

1 

3 

0 

II 

1 

Pipilo  trythrophthdlmus 
(townee) 

$ 

7.88 

9.88 

3.56 

+3.9 

+14.6 

—5.6 

Ageloeus  phaniceus  (red- 
winged  blackbird) 

$ 

9.03 

14.41 

3.61 

+1.6 

+2.1 

+  .6 

Sturnetta  ludoviciana 

(meadow  lark) 

$ 

9.81 

15.70 

2.85 

+6.3 

+3.8 

+10.9 

Sturnella  ludoviciana 

(meadow  lark) 
Quiscalus  purpureus 
(purple  grackle) 
Qtuscalus  purpureus 
(purple  grackle) 
Cyanura  cristata  (blue 

9 
$ 
9 

8.96 
12.19 
11.12 

14.09 
16.64 
14.86 

2.57 
5.22 
4.55 

+6.6 
+3.6 
+3.0 

+2;4 
+6.6 
+6.1 

+9.7 
+1.5 
—1.3 

jay) 

___ 

10.98 

15.11 

5.00 

+6.6 

+11.6 

—2.2 

Colaptes  auratus  (golden- 
winged  woodpecker) 

_ 

11.66 

18.82 

4.40 

+6.8 

+6.0 

—1.1 

Ortyx  virginlanus  (com- 
mon quail) 
Ortyx  virginiamis  (com- 
mon quail) 

$ 
9 

9.46 
9.37 

14.16 
14.02 

2.52 
2.54 

+7.6 
+4.9 

+9.0 

+7.7 

+11.9 
+5.1 

stretch  of  wings,  and  length  of  tail  of  seven  different 
species  of  birds.  In  the  middle  portion  of  the  table  are 
given  the  actual  values  (in  inches)  for  the  Southern 
races,  from  Florida,  whilst  in  the  right  half  are  given 

*  Bull.  Mus.  Comp.  Zool.  Harvard,  vol.  ii.  p.  161-490;  also  Cope's 
"Factors  of  Organic  Evolution,"  p.  45. 


326  THE  EFFECTS  OF  CONDITIONS 

the  percentage  variations,  on  these  values,  of  the  cor- 
responding values  for  the  Northern  races  (from  North- 
ern States,  Massachusetts,  and  Southern  New  Eng- 
land). On  an  average  fifteen  specimens  were  measured 
in  each  case,  the  extreme  numbers  varying  between  6 
and  40.  As  regards  body  length,  we  see  that  the  North- 
ern forms  invariably  exceeded  the  Southern,  the  aver- 
age difference  amounting  to  5.1  per  cent.  In  alar  extent 
they  were  likewise  invariably  greater,  the  average  ex- 
cess being  7.0  per  cent.  In  tail  measurement,  how- 
ever, the  difference  was  not  nearly  so  constant,  it  being 
greater  in  the  Southern  races  than  in  the  Northern  in 
four  out  of  the  ten  sets  of  measurements,  whilst  the 
average  excess  amounted  to  only  2.9  per  cent. 

Accompanying  the  increase  in  size  of  the  Northern 
forms,  Allen  finds  that,  as  a  rule,  there  is  an  apprecia- 
ble decrease  in  colour.  In  the  South,  dark-coloured 
birds,  such  as  the  red-winged  blackbird,  become 
blacker.  The  slaty  and  olive  tints  of  other  birds,  and 
the  various  shades  of  red  and  yellow,  become  far  more 
intense  as  one  proceeds  south,  and  the  pigmentation  of 
the  bill  and  feet  also  increases.  Allen  says  "  the  dif- 
ference in  colour  between  the  extremely  Northern 
and  extremely  Southern  representatives  of  a  given 
species  is  often  so  great  that,  taken  in  connection 
with  other  differences,  as  in  the  general  size  and  the 
size  and  form  of  the  bill,  the  two  extremes  might 
excusably  be  taken  for  distinct  species."  The  size  of 
the  bill  varies,  as  a  rule,  in  the  inverse  ratio  to  the 
size  of  the  body,  and  "  in  many  species  there  is  not 
only  a  marked  relative  increase  in  the  size  of  the  bill 
to  the  southward,  but  in  some  an  absolute  increase, 


OF  LIFE  IN  GENERAL.  327 

especially  in  its  length."  This  increase  is  quite  marked 
in  the  genera  Quiscalus,  Agel&us,  Geothlypis,  Troglo- 
dytes, Seiurus,  etc. 

As  to  the  causes  of  these  geographical  variations,  it 
is  of  course  impossible  to  ascribe  them  with  any  cer- 
tainty even  to  the  indirect  effects  of  change  of  environ- 
ment, much  less  to  the  direct.  Still,  as  Allen  points 
out,  there  is  often  a  somewhat  close  correlation  be- 
tween geographical  varieties  and  the  meteorological 
peculiarities  of  the  regions  in  which  they  occur,  which 
suggests  a  connection  of  some  sort  between  the  two. 
The  increase  in  colour  towards  the  south  coincides 
with  the  increase  in  the  intensity  of  the  sun's  rays,  and 
in  the  humidity  of  the  climate.  The  increase  in  colour 
observed  in  birds  on  passing  from  East  to  West 
seems  also  to  coincide  with  an  increase  of  humidity, 
"  the  darker  representatives  of  any  species  occurring 
where  the  annual  rainfall  is  greatest,  and  the  palest 
where  it  is  least."  This  coincidence  occurs  not  only  in 
the  birds  of  the  United  States,  to  such  a  degree  that 
Allen  says  he  knows  of  no  exception,  but  in  Europe 
also.  Thus  birds  from  the  Scandinavian  coast  are 
very  much  darker  than  in  central  Europe,  where  the 
rainfall  is  only  half  as  great.  Allen  says  that  this  cor- 
relation of  brighter  and  deeper  tint  with  increased 
humidity  is  exhibited  by  the  mammals  of  these  dis- 
tricts, as  well  as  by  the  birds. 

The  differences  in  the  local  races  of  certain  Mam- 
mals are  even  more  striking  than  in  those  of  the  birds. 
The  Canidse,  for  instance,  are  represented  in  North 
America  by  six  species,  viz.,  gray  wolf,  common  fox, 
gray  fox,  coyote,  arctic  fox,  and  kit  fox,  of  which  the 


THE  EFFECTS  OF  CONDITIONS 


first  three  are  the  widest  ranging  species.  Allen 
found  that,  in  respect  of  skull  measurement,  the  com- 
mon wolf  is  fully  a  fifth  larger  in  the  northern  parts  of 
British  America  and  Alaska  than  it  is  in  northern 
Mexico,  the  southern  limit  of  its  habitat,  whilst,  as  we 
see  in  the  accompanying  table,  specimens  from  inter- 
mediate regions  show  a  gradual  intergradation  between 
these  extremes.*  The  common  fox  from  Alaska  is 
about  10  per  cent,  larger  than  that  in  New  England, 


ogo 

I 

8  j 

ggg 

^  £ 

II 

8PECIKS. 

LOCALITY. 

*  en 

:S 

SB! 

3o 

HO 

I 

8 

Forts  Simpson,  Yukon  and 

Gray 
*if 

Rae, 
Forts  Benton  and  Union, 

9 
16 

10.38  in. 
9.45 

5.40  in. 
5.07 

wolf 

Forts  Kearney  and  Harker, 

9 

9.69 

5.18 

Rio  Grande  and  Sonora, 

3 

8.37 

4.31 

Alaska, 

9 

5.98 

3.20 

Common 

Mackenzie  River  District, 

18 

5.80 

3.02 

fox 

Upper  Missouri, 
Essex  County,  New  York, 

9 

12 

5.78 
5.40 

2.90 
2.80 

whilst  the  gray  fox  probably  varies  considerably  more 
in  size  with  locality,  but  the  number  of  skulls  obtained 
for  measurement  (15  in  all)  is  insufficient  to  warrant 
any  generalisation  in  its  case. 

This  increase  in  size  on  passing  from  South  to  North 
is  not  universal,  however.  Thus  lynxes  and  wild  cats, 
though  series  of  skulls  were  obtained  from  such  widely 
separated  localities  as  Alaska,  California,  and  Northern 
Mexico,  revealed  no  appreciable  variation  of  size  with 
*  U.  S.  Geol.  and  Geographical  Survey,  vol.  ii.  p.  309,  1876. 


OF  LIFE  IN  GENERAL.  329 

locality.  Panthers  and  ocelots,  indeed,  showed  a  very 
considerable  increase  in  size  on  passing  southward. 
Still  the  increase  in  size  of  Carnivorous  Mammals  on 
passing  from  South  to  North  may  be  taken  to  be  a 
very  general  rule.  In  addition  to  the  Felidse  men- 
tioned, this  relation  is  well  shown  in  the  badger,  mar- 
ten, wolverine,  and  ermine. 

Of  other  Mammals,  the  relationship  between  locality 
and  size  is  well  shown  by  members  of  the  deer  family, 
the  Virginia  deer  affording  an  especially  striking  in- 
stance. The  Glires  (squirrels,  marmots,  mice)  also 
increase,  as  a  rule,  towards  the  North.  For  instance, 
the  northern  race  of  flying  squirrels  is  half  as  large 
again  as  the  southern,  but  these  two  extremes  are  con- 
nected by  a  complete  chain  of  intermediate  forms. 
As  in  the  case  of  birds,  mere  size  of  body  is  not  the 
only  characteristic  which  varies  with  locality.  The 
ears  and  the  feet  may  undergo  considerable  changes  in 
addition.  Thus  in  mammals  with  large  ears,  such  as 
wolves,  foxes,  some  of  the  deer,  and  especially  the 
hares,  there  is  often  a  striking  increase  in  the  size  of 
these  appendages  on  passing  from  North  to  South. 
The  ears  of  the  little  wood  hare  (Lepus  sylvaticus), 
found  in  Western  Arizona,  are  nearly  twice  the  size 
they  attain  in  the  variety  found  in  more  easterly  and 
northerly  regions.  Again,  in  Lepus  callotis  the  ear  is 
one-third  to  one-fourth  larger  in  examples  obtained 
from  Mexico  than  in  those  from  Wyoming,  whilst  the 
little  brown  hare  (L.  trowbridgei)  shows  a  similar  in- 
crease in  the  size  of  the  ear  in  the  south. 

Darwin  *  has  collected  several  cases  in  which  climate 
*  "  Animals  and  Plants,"  ii.  p.  268. 


330  THE  EFFECTS  OF  CONDITIONS 

had  an  influence  on  the  hairy  covering  of  animals. 
Thus  he  says  "  Dr.  Falconer  states  that  the  Thibet 
mastiff  and  goat,  when  brought  down  from  the  Him- 
alaya to  Kashmir,  lose  their  fine  wool.  At  Angora  not 
only  goats,  but  shepherd-dogs  and  cats,  have  fine  fleecy 
hair,  and  Mr.  Ainsworth  attributes  the  thickness  of  the 
fleece  to  the  severe  winters,  and  its  silky  lustre  to  the 
hot  summers.  Burnes  states  positively  that  the  Kara- 
kool  sheep  lole"  their  peculiar  black  curled  fleeces  when 
removed  into  any  other  country." 

What  may  be  termed,  perhaps,  the  classical  instance 
of  the  formation  of  a  local  race  through  changed  con- 
ditions of  life,  is  that  of  the  Porto  Santo  rabbit.*  A 
female  rabbit  and  her  young  were  turned  loose  on  the 
Island  in  1418,  and  they  increased  so  rapidly  as  to  be- 
come a  nuisance,  and  finally  caused  the  abandonment 
of  the  settlement.  The  present-day  form  of  these  rab- 
bits shows  very  considerable  differences  from  the 
original  form.  Thus  the  two  measured  by  Darwin 
were  only  14r|  and  15  inches  in  length,  instead  of  the 
17  or  18  inches  of  the  English  rabbit.  The  weight 
of  one  of  them — which  had,  however,  become 
somewhat  thin  from  living  in  captivity — was  only 
1  pound  9  ounces,  four  English  wild  rabbits  averaging 
3  pounds  5  ounces.  The  limb  bones  were  smaller  in 
the  proportion  of  rather  less  than  five  to  nine.  In 
colour,  the  Porto  Santo  rabbits  have  a  redder  upper 
surface,  rarely  interspersed  with  any  black  or  black- 
tipped  hairs,  and  in  none  of  the  seven  specimens  exam- 
ined by  Darwin  had  the  upper  surface  of  the  tail  and 
the  tips  of  the  ears  any  of  the  blackish  gray  fur  which 
*  "  Animals  and  Plants,"  i.  p.  118. 


OF  LIFE  IN  GENERAL.  331 

is  generally  regarded  as  one  of  the  specific  characters 
of  the  rabbit.  Finally,  two  male  Porto  Santo  rabbits, 
when  kept  in  captivity,  never  lost  their  extreme  wild- 
ness,  and  would  never  associate  or  breed-  with  the 
females  of  various  breeds  placed  with  them. 

To  what  extent  these  remarkable  changes  were  the 
direct  result  of  a  changed  environment,  it  is,  of  course, 
impossible  to  say;  but  it  was  proved  th^^t  least  the 
colouring  was  a  direct  effect.  Thus  o^^of  the  feral 
rabbits,  after  being  kept  for  four  years  in  captivity, 
was  found  by  Darwin  to  have  acquired  a^jlackish  gray 
fur  on  the  upper  surface  of  the  tail  and  the  edges  of 
the  ears,  whilst  the  whole  body  was  much  less  red;  i.  e., 
it  had  recovered  the  proper  colour  of  its  fur  after  four 
years  of  English  climate. 

The  influence  of  domestication  combined  with  arti- 
ficial selection  is  well  known  to  everyone,  but  what 
shares  of  the  changes  produced  are  to  be  assigned  to 
each  of  these  agencies  is,  as  a  rule,  quite  indeterminable. 
However,  one  may  with  some  reservation  ascribe  to 
domestication  changes  effected  in  characters  which 
have  never  been  the  subject  of  selection.  For  instance, 
the  weight  of  the  rabbit  was  found  by  Darwin  to  in- 
crease on  domestication,  the  result,  probably,  both  of 
more  ample  feeding  and  of  artificial  selection.  The 
skull  capacity,  on  the  other  hand,  by  no  means  propor- 
tionately increased  in  size;  and  as  this  is  scarcely  a 
character  on  which  selection  would  be  practised,  we 
may  consider  the  relative  diminution  as  probably  due 
to  the  direct  influence  of  domestication.  The  reason 
why  we  cannot  say  with  absolute  certainty  that  it  is  a 
direct  effect,  lies  in  the  fact  that  the  character  of 


332 


THE  EFFECTS  OF  CONDITIONS 


"skull  capacity"  may  be  more  or  less  closely  corre- 
lated with  some  other  character  which  has  been  the  ob- 
ject of  selection,  and  so  have  been  thereby  uninten- 
tionally modified.  In  the  accompanying  table  are 
given  the  mean  values  of  Darwin's  measurements:* 


4 

BREED  OP  RABBIT. 

WEIGHT. 

i 

LENGTH  OP 
BODY 
IN  INCHES. 

LENGTH  OP 
SKULL 
IN  INCHES. 

CAPACITY  OP 
SKULL. 

RATIO  OP 
BRAIN  CAPACITY 
TO  LENGTH. 

7  various  wild 
rabbits      . 
3  Porto  Santo 
rabbits      . 
4  various  domestic 
rabbits 
7  large  lop-eared 
rabbits      . 

2  Ib.  15  oz. 
(1  Ib.  9  oz.) 
3  Ib.  4  oz. 
7  Ib.  4  oz. 

17.1 

(14.75) 
19.75 
34.62 

3.09 
2.88 
3.47 
4.11 

950 

828 
864 
1136 

100 

101.1 

78.8 
83.1 

The  weight  of  most  breeds  of  domestic  rabbit  is  not 
much  greater  than  that  of  wild  ones,  but  that  of  the 
lop-eared  variety  is  more  than  twice  as  great.  The 
length  of  body  was  measured  from  incisors  to  anus, 
whilst  the  capacity  of  the  skull  was  determined  by 
weighing  the  small  shot  taken  to  fill  it  (the  numbers 
given  in  the  table  being  the  weight  in  grains).  Tak- 
ing the  relation  of  capacity  of  skull  to  length  of  body 
in  the  wild  rabbit  as  100,  we  see  that,  on  an  average, 
the  skull  capacity  of  the  domestic  rabbit  is  about  20  per 
cent.  less.  That  of  the  Porto  Santo  rabbit  is,  on  the 
other  hand,  very  slightly  greater. 

The  diminution  in  the  size  of  the  rabbit's  brain  is  at- 
tributed  by    Darwin,    to   the    effects    of   disuse,    and 

*  "  Animals  and  Plants,"  i.  p.  133. 


OF  LIFE  IN  GENERAL.  333 

he  ascribes  certain  of  the  changes  in  other  domesticated 
animals  to  a  similar  cause.  Thus  he  found  the  length 
of  the  sternum  in  eleven  different  breeds  of  domestic 
pigeon  to  be  on  an  average  13.0  per  cent,  shorter  than 
in  the  wild  rock  pigeon.*  The  crest  of  sternum, 
scapulae,  and  furculum  were  also  reduced  in  size,  but 
the  wings  were  slightly  increased,  owing  to  the  greater 
length  of  the  wing  feathers.  Again,  in  eight  out  of 
the  eleven  breeds  of  fowl  examined,  the  weight  of 
humerus  and  ulna,  relative  to  that  of  femur  and  tibia, 
was,  on  an  average,  24.2  per  cent,  less  than  in  the  wild 
Gallus  bankiva,  and  in  all  eleven  breeds  the  depth  of 
the  crest  of  the  sternum  (to  which  the  pectoral  muscles 
are  attached),  relative  to  its  length,  was  diminished, 
the  average  diminution  being  17.5  per  cent.f  In  the 
case  of  the  duck,  Darwin  weighed  the  entire  skeleton, 
as  well  as  individual  parts,  and  he  found  that  whilst  in 
the  four  breeds  of  domestic  duck  examined  the  weight 
of  the  femur,  tibia,  and  tarsus,  relative  to  that  of  the 
body,  was,  on  an  average,  28.5  per  cent,  greater  than  in 
the  wild  mallard,  that  of  the  humerus,  radius,  and  meta- 
carpus was  9.0  per  cent.  less.$  That  this  decrease  in 
the  weight  of  the  wing  bones  is  the  direct  result  of  dis- 
use was  proved  by  the  fact  that  in  a  domestic  call  duck 
which  was  in  the  habit  of  constantly  flying  about  for 
miles,  the  relative  weight  of  the  wing  bones  was  actu- 
ally 12.1  per  cent,  greater  than  in  the  wild  mallard. 

*  "Animals  and  Plants,"  i.  p.  184. 
\Ibid.  i.  p.  285.  J/W&,  i.  p.  301. 


PART  III. 

VAKIATION  IN  ITS  KELATIOISr  TO 
EVOLUTION. 

CHAPTER  XI. 

THE    ACTION    OF    NATURAL    SELECTION    ON 
VARIATIONS. 

"Selection  does  nothing  without  variability,  and  this  depends  in  some 
manner  on  the  action  of  the  surrounding  circumstances  on  the 
organism"  (Darwin,  "  Animals  and  Plants"  i.  p.  7). 

' '  The  foundation  of  the  Darwinian  theory  is  the  variability  of  species  " 
(Wallace,  "Darwinism," p.  41). 

"  What  forms  the  basis  of  the  constant  'individual  variations1  which, 
after  the  precedent  of  Darwin  and  Wallace,  we  regard  as  the  foun- 
dation of  all  pi'ocesses  of  natural  selection  ?  "  ( Weismann,  "  Oerm- 
Plasm,"p.  410.) 

Proof  of  Natural  Selection  in  the  crab  and  in  the  sparrow— Selection 
in  man — Evolution  of  the  mouse — Inheritance  of  acquired  char- 
acters seems  to  be  shown  by  cumulative  effects  of  conditions  of 
life,  as  European  climate  acting  on  American  maize;  domestica- 
tion acting  on  wild  turkeys  and  ducks ;  changed  climate  acting  on 
sheep  and  dogs— Environment  may  act  on  germ- plasm  through 
specific  excretions  and  secretions — Cases  of  inherited  effects  of  use 
and  disuse,  and  of  epilepsy,  accounted  for — Somatic  variations 
may  increase  variability,  and  so  afford  Natural  Selection  a  better 
handle  to  work  upon. 

THE  contents  of  this  chapter  are  well  summarised  in 
the  three  quotations  given  at  its  head.  It  deals  with 
variations  in  their  relation  to  Natural  Selection,  and 


336  ACTION  OF  NATURAL  SELECTION 

with  the  gradual  evolution  thereby  brought  about. 
The  fundamental  importance  of  variations  in  the  evo- 
lutionary process  has  been  dwelt  on  again  and  again  by 
Darwin,  by  Wallace,  and  by  most  of  the  subsequent 
writers  on  the  subject,  and  as  this  doctrine  is  so  uni- 
versally admitted,  it  is  unnecessary  to  discuss  it  any 
further  here.  At  the  present  day,  however,  there  ap- 
pears to  be  a  considerable  amount  of  scepticism  among 
some  men  of  science  as  to  the  extreme  importance 
which  has  been  generally  attached  to  the  agency  of 
Natural  Selection.  Some,  such  as  Driesch,  have  even 
denied  its  existence  altogether,  whilst  many  others  hold 
that  its  existence  has  never  been  demonstrated.  They 
hold  with  Lord  Salisbury  *  that  "  no  man,  as  far  as 
we  know,  has  ever  seen  it  at  work."  The  evidence 
to  be  adduced  will  show,  I  believe,  that  this  statement 
is  erroneous,  but  even  if  it  be  correct,  it  cannot  dis- 
prove the  theory,  the  validity  of  which  seems  to  me  a 
logical  necessity.  Thus,  granted  the  geometrical  rate 
of  increase  possessed  by  all  organisms,  and  the  severe 
struggle  for  existence  thereby  entailed;  granted  that 
all  organisms  show  individual  variations,  and,  to  a  con- 
siderable extent,  hereditary  transmission  of  such  varia- 
tions, then  it  must  follow  that,  on  an  average,  more  of 
the  organisms  possessing  favourable  variations  better 
adapted  to  their  environment  will  survive  than  of  those 
possessing  less  favourable  ones.  That  is  to  say,  the 
species  will  become  gradually  modified  by  the  action  of 
Natural  Selection. 

Numerical   evidence   in  support   of  the   theory   of 
Natural    Selection    has    been    obtained    only    quite 
*  "Presidential  Address,  British  Association,"  1894. 


ON  VARIATIONS.  337 

recently,  and  this  is  not  to  be  wondered  at,  considering 
the  numerous  and  extended  observations  it,  as  a  rule, 
entails.  In  the  case  of  the  small  shore  crab,  Carcinus 
mcznas,  however,  Professor  Weldon  *  has  succeeded  in 
overcoming  most  of  the  inevitable  difficulties  and  pit- 
falls, and  has  obtained  evidence  which,  though  at 
present  not  absolutely  convincing,  yet  has  a  very  high 
degree  of  probability  of  truth.  In  1893  Mr.  H. 
Thompson  carefully  determined  the  relation  of  the 
mean  frontal  breadth  to  the  carapace  length  in  male 
crabs  collected  at  a  particular  patch  of  beach  in 
Plymouth  Sound.  The  mean  breadth  was  found  to 
vary  very  rapidly  with  the  length  of  the  crab,  hence  its 
value  was  determined  separately  in  small  groups  of 
crabs,  differing  from  each  other  by  not  more  than 
.2  mm.  Twenty-five  such  groups,  for  crabs  between 
10  and  15  mm.  long,  were  measured  in  respect  of  fron- 
tal breadth.  A  similar  series  of  measurements  was 
carried  out  by  Thompson  on  crabs  collected  at  the  same 
spot  in  1895,  and  another  by  Weldon  on  crabs  collected 
in  1898.  On  comparing  the  three  series  of  data  thus 
obtained,  it  was  evident  that  the  mean  breadth  of  crabs 
of  a  given  carapace  length  had  steadily  decreased.  For 
instance,  in  crabs  with  a  carapace  length  of  11.5  mm., 
the  frontal  breadth  had  a  percentage  length  of  79.72 
in  1893,  78.88  in  1895,  and  78.40  in  1898.  Again,  in 
14  mm.  crabs,  it  had  a  length  of  76.26  in  1893,  75.44 
in  1895,  and  74.44  in  1898. 

It  would  seem,  therefore,  that  the  frontal  breadth  of 
these  crabs  is  diminishing,  year  by  year,  at  a  very  rapid 
rate.     This  Professor  Weldon  attributes  to  a  selective 
*  Report  of  Brit.  Assn.,  1898,  p.  887. 


338  ACTION  OF  NATURAL  SELECTION 

destruction,  caused  by  certain  rapidly  changing  condi- 
tions in  Plymouth  Sound.  Owing  to  the  building  of  a 
huge  breakwater,  the  scour  of  the  tide  has  been  dimin- 
ished, and  the  large  quantities  of  china  clay  carried 
down  by  the  rivers  from  Dartmoor  into  the  Sound 
therefore  settle  in  increasing  quantities  in  the  Sound 
itself.  Also  the  quantity  of  sewage  and  refuse  finding 
its  way  into  the  Sound  is  steadily  increasing,  owing  to 
the  increase  in  the  size  of  the  contiguous  towns  and 
dockyards.  "  It  is  well  known,"  says  Professor  Wei- 
don,  "  that  these  changes  in  the  physical  condition  of 
the  Sound  have  been  accompanied  by  the  disappearance 
of  animals  which  used  to  live  in  it2  but  which  are  now 
found  only  outside  the  area  affected  by  the  break- 
water." In  order  to  test  his  supposition  of  selective 
destruction,  Professor  Weldon  placed  a  number  of 
crabs  in  a  large  vessel  of  sea  water,  in  which  a  consider- 
able quantity  of  very  fine  china  clay  was  suspended. 
The  clay  was  prevented  from  settling  by  a  slowly  mov- 
ing automatic  agitator.  After  a  time,  the  dead  crabs 
were  separated  from  the  living,  and  both  were  meas- 
ured. In  the  figure  given  below  is  shown  the  result 
obtained. 

Here  the  upper  curve  shows  the  distribution  of  fron- 
tal breadths  of  the  248  male  crabs  experimented  on,  and 
the  dotted  curve  the  distribution  of  frontal  breadths 
of  the  94  survivors.  The  line  0  represents  the  mean 
frontal  breadth  of  all  the  crabs,  the  dotted  line  S  the 
mean  of  the  survivors,  and  the  dotted  line  D  the  mean 
of  the  dead  crabs.  The  crabs  which  survived  thus  had 
a  distinctly  smaller  frontal  breadth  than  those  which 
were  killed,  just  as  the  1898  crabs  had  a  smaller 


ON  VARIATIONS. 


339 


breadth  than  the  1895  ones,  and  these  than  the  1893 
ones.  There  seems  no  reason  to  believe  that  the  action 
of  the  mud  upon  the  beach  is  different  from  that  in  an 
experimental  aquarium,  and  hence,  in  Professor  Wei- 
don's  opinion,  there  is  "  no  escape  from  the  conclusion 
that  we  have  here  a  case  of  Natural  Selection  acting 
with  great  rapidity  because  of  the  rapidity  with  which 
the  conditions  of  life  are  changing."  The  selective  de- 
struction seems  to  depend  on  the  nitration  of  the  water 
into  the  gill  chambers  of  the  crabs.  To  quote  Pro- 
fessor Weldon,  "  The  gills  of  a  crab  which  has  died 
during  an  experiment  with  china  clay  are  covered  with 
fine  white  mud,  which  is  not  found  in  the  gills  of  the 


s  D 


30 

20 
10 

A 

A 

~2_ 

^ 

- 

^> 

i  —  ^ 

X, 

--A 

^M 

—  

0 

FIG.  26.— Distribution  of  the  frontal  breadths  of  248  male  crabs, 
and  of  the  94  survivors. 

survivors.  In  at  least  90  per  cent,  of  the  cases  this 
difference  is  very  striking."  Professor  Weldon  thinks 
it  can  be  shown  that  a  narrow  frontal  breadth  renders 
one  part  of  the  process  of  filtration  of  water  more  effi- 
cient than  it  is  in  crabs  of  greater  frontal  breadth. 

Such,  then,  is  Professor  Weldon's  demonstration  of 
a  particular  instance  of  Natural  Selection.  In  order 
to  strengthen  the  proof  of  its  existence,  further  meas- 
urements of  crabs  collected  at  the  same  spot  a  few  years 


340  ACTION  OF  NATURAL  SELECTION 

hence  ought  to  be  made,  as  Professor  Weldon  him- 
self well  recognises,  in  order  to  see  whether  the  de- 
structive process  is  still  continuing.  If  this  is  the  case, 
and  if  crabs  measured  in,  say,  1903  and  1908  show  a 
further  diminution  of  frontal  breadth,  then  the  evi- 
dence in  favour  of  selection  would  amount  to  a  very 
high  degree  of  probability  indeed.  Owing  to  the 
changing  relation  of  its  parts  with  growth,  the  crab  is 
a  somewhat  unsatisfactory  organism  to  work  with,  and, 
indeed,  the  apparent  change  observed  between  1893 
and  1898  might  possibly,  though  not  probably,  owe  its 
origin  to  quite  another  cause  than  Selection.  For  in- 
stance, the  conditions  of  environment  such  as  tempera- 
ture, nutrition,  and  purity  of  the  water  may  have  acted 
directly  on  the  crabs  so  as  to  retard  their  growth. 
Now  Professor  "VYeldon  assumes  that  all  crabs  of,  say, 
12  mm.  length  are  approximately  the  same  age,  but  ob- 
viously this  need  not  be  so  from  year  to  year.  Under 
less  favourable  conditions,  the  moulting  may  have  gone 
on  as  usual,  but  the  rate  of  growth  have  been  reduced. 
Now  we  have  seen  that  the  frontal  breadth  diminishes 
very  rapidly  with  growth,  and  hence  it  might  happen 
that  the  narrower  fronted  12  mm.  crabs  of  1898  are 
narrower  simply  because  they  are  older  than  were  the 
more  favourably  situated  12  mm.  crabs  of  1893.  Mr. 
J.  T.  Cunningham  *  has  pointed  out  that  in  1893  the 
temperature  of  the  Channel  waters  was  abnormally 
high,  and  he  considers  that  this  produced  a  more  rapid 
growth  of  the  crabs,  and  hence,  for  a  given  size  of  crab, 
an  apparent  increase  of  frontal  breadth.  However, 
Professor  Weldon  f  does  not  believe  that  the  tempera- 
*  Nature,  vol.  Iviii.  p.  593.  \Ibid.,  p.  595. 


ON  VARIATIONS.  341 

ture  of  the  beach  where  his  crabs  were  collected,  in 
that  it  looks  due  south  and  is  uncovered  for  hours  daily, 
was  any  lower  in  1898  than  in  1893,  and  also  he  found 
that  crabs  gathered  in  January  were  no  narrower 
fronted  than  those  gathered  in  August,  as  they  ought 
to  have  been  on  Cunningham's  hypothesis. 

The  proof  of  the  existence  of  Natural  Selection 
really  centres  upon  the  proof  of  a  selective  destruc- 
tion or  death  rate.  If  among  any  group  of  organisms 
the  eliminated  individuals  can  be  measured  and  exam- 
ined, as  well  as  the  survivors,  and  if  it  be  found  that 
these  two  divisions  differ  in  their  mean  characters,  then 
Natural  Selection  must  have  been  at  work.  Very 
likely  the  parts  or  organs  measured  do  not  represent 
the  characters  upon  which  the  selective  process  had 
been  acting,  but  are  merely  correlated  with  them. 
But  that  is  no  matter.  The  offspring  of  the  survivors 
will  have  different  average  qualities  from  those  of  the 
previous  unselected  generation,  or  the  race  will  be- 
come modified  by  Natural  Selection. 

Unfortunately  in  the  majority  of  cases,  as  in  Pro- 
fessor Weldon's  crabs,  it  is  impossible  to  get  hold  of 
the  eliminated  individuals,  and  hence  the  proof  of 
Natural  Selection  is  rendered  much  more  laborious, 
and  at  the  same  time  more  open  to  possible  source  of 
error.  In  the  case  of  the  (introduced)  English  spar- 
row (Passer  domesticus),  however,  Bumpus  *  has  suc- 
ceeded in  obtaining  the  desired  material.  One  hun- 
dred and  thirty-six  of  these  sparrows  were  collected 
after  a  very  severe  storm  of  snow,  rain,  and  sleet  in 
North  America,  and  of  these  72  revived,  whilst  64 
*  Biol.  Lectures,  Wood's  Holl,  1898,  p.  211. 


342 


ACTION  OF  NATURAL  SELECTION 


perished.  On  comparing  the  survivors  with  the  elimi- 
nated individuals,  very  appreciable  differences  were 
found  to  exhibit  themselves.  The  means  of  the  values 
obtained  with  all  the  birds,  both  male  and  female,  are 
given  in  the  accompanying  table : 


MEAN  VALUES. 

ARITHMETIC  MEAN  ERROR. 

i 

i 

.  H 

GQ 

g 

i 

0 

H 

HH 

0 

H 

H  W 

1 

g 

o  PS 

E 

•^ 

B 

Q 

g  fc 

B 

H 

tf      ^ 

i 

HI 

a 

H 

Total  length, 
Alar  extent, 

158  mm. 
245mm. 

160  mm. 
245mm. 

+1.27 
±0.0 

2.51 

4.20 

3.48 
4.60 

+38.6 
+  9.5 

Weight, 
Length  of  beak  &  head 
Length  of  humerus, 
Length  of  femur, 
Length  of  tibio-tarsus 

25.2  gm. 
31.6  mm. 
.736  inch 
.716    " 
1.138    " 

25.8  gm. 
31.5  mm. 
.728  inch 
.709     " 
1.128     " 

+2.38 
—  .32 
-1.09 
-  .98 
—  .88 

10.9 
5.51 
.016 
.014 
.0294 

12.6 
5.64 
.0201 
.020 
.0338 

+15.6 
+  2.4 
+25.6 
+42.9 
+15.0 

Width  of  skull, 

.603    •• 

.601     " 

—  .33 

.010 

.012 

+20-0 

Length  of  sternum, 

.845    " 

.834    •* 

—1.30 

.032 

.038 

-  3.1 

Here  we  see  that  the  average  characters  differ  but 
little.  The  eliminated  individuals  are  1.27  per  cent, 
greater  in  length,  and  2.38  per  cent,  greater  in 
weight,  whilst  they  are  about  1  per  cent,  smaller 
than  the  survivors  in  respect  of  most  of  the  other 
characters  measured.  The  variability,  or  range  of 
variation  of  the  eliminated  birds  about  their  mean, 
is,  however,  very  much  greater  than  in  the  case  of  the 
survivors.  Of  the  nine  characters  measured,  the  varia- 
bility is  greater  in  eight,  the  average  excess  being  no 
less  than  18.8  per  cent.  The  variability  was  less  in  re- 
spect of  the  sternum  alone,  and  then  only  by  3.1  per 
cent.  In  the  accompanying  figure  are  given  curves  of 
distribution  of  the  lengths  of  the  surviving  and  of  the 
eliminated  birds.  It  is  obvious  that  the  dotted  line 


ON  VARIATIONS. 


343 


curve,  which  represents  the  eliminated  individuals,  is, 
on  the  whole,  much  more  flat-topped  than  the  other 
curve.  The  very  long  individuals  seem  especially 
handicapped  in  the  struggle  for  existence,  as  of  the  18 
birds  obtained  in  which  the  length  was  164  mm.  and 
upwards,  no  less  than  14  perished.  Also  the  two 
shortest  birds  obtained  perished.  The  conclusion 
which  Bumpus  draws  from  these  most  interesting  ob- 
servations is  as  follows :  "  Natural  Selection  is  most 
destructive  of  those  birds  which  have  departed  most 
from  the  ideal  type,  and  its  activity  raises  the  gen- 
eral standard  of  excellence  by  favouring  those  birds 
which  approach  the  structural  ideal."  The  observa- 
tions really  show  more  than  this,  however.  It  is 

15 


40 


\ 


3M 


256       B8 


150        153        154  -     i'56      J58      JI6Q  _ 

Length  of  birds  in  millimeters. 

FIG.  27.—  Distribution  of  the  lengths  of  surviving  and  of 
eliminated  sparrows. 

true  that,  as  a  rule,  the  most  extreme  individuals  in 
either  direction  are  eliminated,  but  if  the  distributions 
of  the  various  characters  be  plotted  out  as  in  the 
above  figure,  it  will  be  seen  that  in  the  case  of  some 
of  the  other  characters,  as  in  that  of  length,  the  elimi- 
nating process  acts  much  more  on  the  extreme  in- 
dividuals in  one  direction  than  on  those  in  the  other. 


344 


ACTION  OF  NATURAL  SELECTION 


For  instance,  the  accompanying  figure  shows  the  distri- 
bution of  the  weight  values  of  the  birds.  The  curves 
are  very  irregular,  but  it  is  obvious  that  the  dotted  line 
curve  is  shifted  distinctly  to  the  right,  indicating  that 
the  eliminated  birds  were,  on  an  average,  heavier.  This 
conclusion  has  already  been  obtained  by  the  simple 
process  of  taking  averages;  but  the  curves  show  in  ad- 
dition that  it  is  the  very  heavy  birds  which  were  more 


115 


lo 


•8  5 


V 


24 


25          26  27          28          29 

Weight  of  birds  in  grams 


30 


31 


32 


FIG.  28. — Distribution  of  the  weights  of  surviving  and  of 
eliminated  sparrows. 

especially  eliminated.  Thus  of  the  14  birds  of  27.3 
gms.  and  upwards  obtained,  only  three  survived. 
Similarly  also  in  respect  of  the  femur  measurements, 
it  was  found  that  of  the  19  birds  obtained  with  a  femur 
length  of  .685  inch  or  less,  only  7  survived  whilst  12 
were  eliminated. 

The  next  generation  of  birds  collected  in  tEe  storm- 
swept  area  would  accordingly  be  shorter  in  length, 
weigh  less,  have  longer  legs,  have  a  longer  sternum  and 
a  greater  brain  capacity  than  the  former  generation; 
supposing,  of  course,  that  the  variations  existing  in 


ON  VARIATIONS.  345 

these  characters  were  partly  of  blastogenic,  and  not 
wholly  of  somatogenic  origin;  and  this  could  scarcely 
fail  to  be  the  case.  Several  of  the  changes  in  char- 
acters, especially  of  the  total  length,  weight,  and  femur 
length,  might  possibly  be  present,  on  an  average,  to  just 
as  marked  an  extent  as  in  their  parents  (the  survivors 
of  the  previous  generation);  for  though  the  characters 
would  tend  to  undergo  reduction  by^  virtue  of  their 
"  regression  towards  mediocrity,"  yet  they  would 
tend  to  be  enhanced  by  reason  of  the  fact  that  more 
of  the  extreme  individuals  (which  would  be  of  propor- 
tionally greater  weight  in  determining  the  characters 
of  the  next  generation)  had  been  weeded  out  than 
of  the  mediocre  ones.  Bumpus  does  not  give  any 
details  as  to  the  way  in  which  the  sparrows  were 
collected,  and  whether  the  sample  obtained  was  repre- 
sentative of  all  the  sparrows  in  the  area  in  question. 
Supposing  it  were  not,  and  it  included  only  spar- 
rows which  were  exposed  to  the  force  of  the  storm 
through  failing  to  get  adequate  shelter,  then,  of  course, 
the  average  change  produced  in  the  characters  of  the 
next  generation  would  be  much  less  than  that  suggested 
by  the  above  figures. 

Professor  Weldon  *  has  adopted  a  very  ingenious 
method  for  determining  the  presence  or  absence  of  Nat- 
ural Selection  in  the  case  of  a  certain  terrestrial  mol- 
lusc, Clausilia  laminata.  The  outer  layer  of  the  shell  in 
this  and  other  pulmonates  is  secreted  by  the  growing 
edge  of  the  mantle  once  and  for  all,  and  it  undergoes 
practically  no  subsequent  change.  The  upper  whorls 
of  an  adult  shell  therefore  afford  an  unaltered  record 
*Biometrika,  i.  p.  109,  1901 


346  ACTION  OF  NATURAL  SELECTION 

of  the  condition  of  the  young  shell,  from  which  this 
adult  was  formed  by  the  subsequent  deposition  of  new 
material.  By  measuring  the  upper  whorls  of  the  adult 
shells,  one  is  accordingly  able  to  determine  the  char- 
acters possessed,  not  by  all  young  shells,  but  by  the 
young  shells  which  were  successful  in  attaining  the 
adult  condition.  How  would  measurements  on  such 
adult  shells  compare  with  those  on  young  and  growing 
shells,  some  of  which  would  almost  certainly  undergo 
destruction  before  attaining  their  full  development? 
To  answer  this  question,  Professor  Weldon  measured 
with  great  exactness  the  radius  of  the  spiral  at  various 
(angular)  distances  from  the  apex  of  the  shell  in  100 
adult  individuals,  and  also  in  100  young  individuals  of 
less  than  half  their  length.  The  means  of  the  values 
so  obtained  were  practically  identical  in  the  two  classes 
of  shells,  so  it  seems  to  follow  that  the  mean  spiral  of 
young  shells  is  not  altered  during  growth  by  any  process 
of  selective  destruction.  On  the  other  hand,  the  varia- 
bility of  the  radial  spiral  measurements  was  consider- 
ably greater  in  the  young  shells  than  in  the  adult  ones 
(on  an  average,  in  the  proportion  of  120  to  100  for  the 
first  whorl  and  a  half.)  Hence  we  may  conclude  that 
during  the  growth  of  this  mollusc  some  processes  are  at 
work  which  effectually  eliminate  the  abnormal  shells 
more  rapidly  than  the  others,  and  so  diminish  the 
variability  of  the  survivors.  As  the  average  character 
of  the  race  does  not  undergo  any  change,  it  follows  that 
the  abnormalities  eliminated  are  evenly  distributed 
about  the  mean.  Such  a  process  of  selection  has  been 
termed  by  Professor  Pearson  *  periodic. 
*"  Grammar  of  Science,"  p.  413. 


ON  VAEIATIONS,  347 

The  proof  given  by  Professor  Pearson  *  of  the  exist- 
ence of  a  selective  death  rate  in  the  case  of  man  seems 
to  me  scarcely  to  entitle  him  to  speak  of  it  as  a  case  of 
"  Natural  Selection."  Thus  he  shows  that  there  is  a 
fairly  close  correlation  (r  =  .26)  between  the  ages  at 
death  of  brother  and  brother,  and  a  less  close  one  be- 
tween those  of  fathers  and  sons  (r  —  .12  to  .14).  For 
instance,  the  mean  age  at  death  of  men  not  dying  as 
minors  is  61  years.  If,  however,  one  brother  of  a  pair 
dies  at  25,  then  the  other  will,  on  an  average,  die  at 
51.6,  or  9.4  years  earlier  than  the  mean;  if  one  dies  at 
85,  then  the  other  will,  on  an  average,  die  at  67.2,  or  6.2 
years  later.  There  is  something  in  the  constitution  of 
a  man,  therefore,  which  to  a  large  extent  determines 
when  he  shall  die,  or  undergo  elimination.  His  death 
is  not  at  all  a  matter  of  chance.  Further  analyses  of 
data  by  Miss  Beeton  and  Professor  Pearson  f  indicate 
the  same  thing,  though  they  also  lead  to  other  and  some- 
what unexpected  conclusions.  Thus,  from  the  pedigree 
records  of  members  of  the  society  of  Friends,  the  au- 
thors found  that  elder  (adult)  brothers  and  sisters  on  an 
average  live  distinctly  longer  than  younger  (adult) 
brothers  and  sisters,  and  that  the  greater  the  difference 
in  age,  the  greater  the  difference  in  expectation  of  life. 
For  instance,  a  man  born  6  years  after  his  elder  brother 
will  probably  live  4  years  less  than  he  will;  one  born 
10  years  after,  7  years  less,  and  one  born  17  years  after, 
as  much  as  12  years  less.  Put  in  other  words,  the 
eldest  children  of  a  family  have  the  best  chance  of  life, 

*"  Grammar  of  Science,"  p.  497;  also  Beeton  and  Pearson,  Proc. 
Roy.  Soc.,  Ixv.  p.  290,  1899. 
fBiometrika,  i.  p.  50,  1901. 


348  ACTION  OF  NATUEAL  SELECTION 

and  the  youngest  the  worst.  We  may  perhaps  look 
upon  this  decrease  of  vitality  as  the  direct  result  of  the 
diminished  vigour  of  the  parents  at  the  time  of  con- 
ception, and  of  the  mother  during  the  period  of  intra- 
uterine  development.  If  this  is  actually  the  case,  then 
we  ought  to  find  that  the  expectation  of  life  is  more 
closely  correlated  with  the  age  of  the  mother  at  the 
time  of  conception  than  with  that  of  the  father. 

Again  Miss  Beeton,  in  conjunction  with  Mr.  G.  U. 
Yule  and  Professor  Pearson,*  have  found  that  there  is 
a  direct  correlation  between  the  duration  of  life  in 
parents,  and  the  number  of  children  borne  by  them. 
It  was  found  that  fertility  was  correlated  with 
longevity  even  in  parents  of  50  years  and  upwards, 
when  the  fecund  period  is  passed,  though  the  correla- 
tion is  not  by  any  means  so  close  as  in  parents  under 
50. 

For  instance,  American  mothers  dying  at  25  had  on 
an  average  2.2  children;  those  at  35,  4  children;  and 
those  at  50,  5.7  children:  but  mothers  dying  at  TO  had 
on  an  average  6.8  children,  and  those  at  90,  no  less 
than  7.6  children.  "With  English  mothers  dying  at  50 
years  and  over,  the  increased  fertility  is  not  so  marked, 
and  it  becomes  slightly  diminished  in  those  living  over 
75  years.  Similarly  also  with  English  fathers  the  re- 
lation of  fertility  to  longevity  is  less  marked  than  in 
the  case  of  American  fathers. 

All  these  data  may  undoubtedly  be  taken  to  indi- 
cate, therefore,  that  longevity  is  inherited  in  man,  and 
as  long  life  means  a  healthier  and  stronger  constitu- 
tion, it  is  natural  to  find  that  it  also  betokens  increased 
*Proc.  Roy.  Soc.,  Ixvii.  p.  159,  1900. 


ON  VARIATIONS.  349 

power  of  procreating  offspring.  It  does  not  neces- 
sarily follow,  however,  that  Natural  Selection,  in  the 
ordinary  sense  of  the  term,  is  at  work.  The  time  of 
death  may  be  quite  uncorrelated  with  any  particular 
structural  characters  of  the  body,  but  be  dependent 
only  on  the  so-called  vigour  or  vitality  of  the  organism. 
Each  subsequent  generation  may  therefore  be  more 
"  vigorous  "  than  the  one  before  it,  owing  to  the  elimi- 
nation of  a  portion  of  the  less  vigorous  individuals,  but 
as,  in  all  probability,  there  is  always  a  tendency  to  the 
production  in  each  generation  of  a  certain  number  of 
unfit  individuals,  or  a  slight  diminution  in  the  average 
vitality  of  the  whole  group,  it  would  follow  that  a  cer- 
tain amount  of  elimination  is  always  necessary,  to  en- 
able a  race  to  maintain  its  average  vitality  from  one 
generation  to  the  next.  Certainly,  in  the  case  of  the 
human  race,  there  is  no  evidence  that  the  average 
vigour  and  vitality  are  increasing.  Everything  goes 
to  prove  rather  that  they  are  on  the  wane. 

Man  is  therefore  an  unsatisfactory  organism  in  which 
to  determine  either  the  existence  or  the  non-existence 
of  Natural  Selection.  His  conditions  of  death  are  as 
unnatural  as  his  conditions  of  life.  The  usual  cause 
of  his  death,  disease,  counts  for  little  or  nothing 
amongst  the  lower  animals,  whilst  the  usual  causes  of 
death  amongst  them,  namely,  want  of  food  and  natural 
enemies,  count  for  little  or  nothing  with  man.  To 
prove  the  existence  of  Natural  Selection,  one  must 
choose  for  observation  an  organism  living  under  nat- 
ural conditions. 

A  very  interesting  case  of  the  formation  of  a  local 
race  through  the  probable  agency  of  Natural  Selection 


350  ACTION  OF  NATUEAL  SELECTION 

has  recently  been  described  by  H.  L.  Jameson.*  On 
the  north  side  of  Dublin  Bay  there  is  a  tract  of  sand- 
hills, running  along  the  coast  for  about  three  miles.  It 
is  separated  from  the  mainland  by  a  tidal  channel  about 
a  quarter  of  a  mile  wide  at  high  water,  but  only  20 
yards  or  so  at  low  water.  These  sandhills  are  thickly 
populated  with  mice,  which  were  noticed  by  Jameson 
to  harmonise  strikingly  in  colour  with  the  sand.  Traps 
were  set,  and  altogether  36  mice  were  caught.  The 
specimens  varied  considerably  in  the  shade  of  their 
fur,  showing  every  gradation  from  the  typical  Mus 
musculus  of  the  farmhouses  in  Ireland  and  England  to 
individuals  with  extremely  pale  dorsal  fur — usually  of 
a  rufous  or  fulvous  gray  colour — pale  buff  ventral  sur- 
face, and  pale  and  fulvous  appearance  of  the  hairs  on 
the  ears,  tail,  and  other  parts  of  the  body.  Also  the 
feet  were  white  or  pale  buff,  instead  of  the  smoky  gray 
or  white  of  the  ordinary  House-mouse,  whilst  the  claws 
were  flesh-coloured.  Of  the  36  specimens,  only  five 
were  of  more  or  less  the  typical  colour,  four  were 
slightly  paler,  and  the  remaining  27  markedly  palles- 
cent. 

These  mice  differ  in  other  characters  also.  Thus,  if 
the  adult  specimens  be  split  up  into  three  groups,  ac- 
cording to  their  colouration,  and  means  taken  of  the 
measurements  made  by  Jameson,  the  values  given  in 
the  table  below  are  obtained.  Though  the  number 
of  measurements  is  so  small,  there  can  be  little 
doubt  that  the  tail  of  the  palest  individuals  is 
distinctly  longer  than  that  of  the  typical  ones.  Per- 
haps also  the  head  and  body  and  the  hind  foot  are 
*  Journ.  Linn.  Soc.  (Zool),  vol.  xxvi.  p.  465,  1898. 


slightly  shorter,  though  one  cannot  speak  with  any  cer- 
tainty. The  habits  of  the  mice  had  changed  in  addi- 
tion, as  they  were  found  to  burrow  their  own  holes,  no 
holes  burrowed  by  other  animals  being  available,  as  in 
the  case  of  the  typical  wild  mouse. 

The  development  of  the  protective  colouration  and 
habits  probably  owes  its  origin  to  the  short-eared  owls 
and  hawks  which  were  noticed  to  frequent  the  sand- 
hills, and  which  would  more  readily  perceive  and  cap- 
ture the  darker  mice.  These  would  gradually  be 
weeded  out,  therefore,  whilst  the  light-coloured  indi- 
viduals would  survive  and  propagate  their  more  favour- 
able characteristics. 


LENGTH  IN  MILLIMETRES  OF 

NUMBER    OF 

MEASURES. 

Head  and  Body. 

Tail. 

Hind  Foot. 

7 

Typical  or  slightly 

pale 

81.3 

76.5 

18.1 

9 
20 

Markedly  pale 
Still  more  pale 

76.3 

80.2 

77.8 
80.0 

16.9 

17.2 

Perhaps  the  most  interesting  point  of  all  about  these 
observations  is  that  it  has  been  found  possible  to  fix  a 
time  limit  for  the  duration  of  the  evolutionary  process. 
The  sandbanks  are  known  to  be  gradually  increasing  in 
area,  and,  by  a  careful  study  of  old  maps,  Jameson 
found  that  previous  to  1780  they  did  not  exist  at  all. 
In  1810  the  island  was  only  a  quarter  of  a  mile  long, 
so  we  may  conclude  that  the  pale  race  of  mice  has  had 
not  more  than  about  a  hundred  years  for  its  evolution. 

Are  Acquired  Characters  Inherited?    We  see  that 


352  ACTION  OF  NATURAL  SELECTION 

Evolution  is  brought  about  by  the  action  of  Natural 
Selection  on  variations,  it  selecting  some  and  rejecting 
others,  and  so  gradually  altering  the  average  char- 
acters of  the  race;  but  are  blastogenic  or  germinal 
variations  alone  of  value  to  such  a  selective  agency, 
and  are  somatic  variations,  or  so-called  acquired  char- 
acters, valueless  in  this  respect  ?  As  is  well  known,  the 
question  of  the  heritableness  of  acquired  characters  has 
been  one  of  the  most  hotly  debated  of  all  biological 
problems,  and  is  one  which  even  now  separates  most 
biologists  into  two  opposite  and  apparently  irreconcil- 
able camps.  It  behoves  us,  therefore,  to  see  if  we  can- 
not find  some  via  media,  which,  though  unable  to  ad- 
mit of  the  heritableness  of  localised  tissue  changes 
such  as  injuries  and  mutilations,  is  yet  able  to  adopt 
reasonable  evidence,  both  experimental  and  theoretical, 
in  favour  of  a  partial  inheritance  of  certain  general- 
ised tissue  changes. 

The  chief  experimental  evidence  in  support  of  the 
apparent  heritableness  of  acquired  characters  lies  in  the 
numerous  and  undoubted  proofs  of  the  cumulative 
action  of  conditions  of  life.  Of  such  proofs,  one  of  the 
most  striking  is  that  recorded  by  Darwin  *  with  refer- 
ence to  the  effects  of  a  European  climate  on  the  Ameri- 
can varieties  of  maize.  Thus  Metzger  cultivated  in 
Germany  a  tall  kind  of  maize,  Zea  altissima,  brought 
from  the  warmer  parts  of  America,  and,  "  During  the 
first  year  the  plants  were  twelve  feet  high,  and  a  few 
seeds  were  perfected.  .  .  In  the  second  generation  the 
plants  were  from  nine  to  ten  feet  in  height,  and 
ripened  their  seed  better.  .  .  Some  of  the  seeds  had 
*  "  Animals  and  Plants,"  i.  p.  340. 


ON  VARIATIONS.  353 

even  become  yellow,  and  in  their  now  rounded  form 
they  approached  common  European  maize.  In  the 
third  generation  nearly  all  resemblance  to  the  original 
and  very  distinct  American  parent-form  was  lost.  In 
the  sixth  generation  this  maize  perfectly  resembled  a 
European  variety." 

Other  instances  of  the  cumulative  effects  of  condi- 
tions of  life  on  plants  have  already  been  recorded  in 
former  chapters.  Thus  Lesage  found  that  if  Garden 
cress  were  treated  with  salted  water,  a  much  more 
marked  effect  was  produced  in  the  second  year  than  in 
the  first,  the  alteration  effected  in  the  tissues  of  the 
second  generation  seeming  to  be  carried  on  from  the 
point  gained  in  the  first.  Bonnier  found  that  seeds  of 
Teucrium  scorodonia  gathered  from  plants  growing  at 
high  altitudes,  and  sown  in  Paris,  only  produced  plants 
showing  nearly  similar  characters  to  the  local  forms 
after  three  years'  exposure  to  the  new  environment. 

Among  members  of  the  Animal  Kingdom  the  evi- 
dence is  no  less  conclusive.  Thus  Darwin  *  records 
that  "  Dr.  Bachman  states  that  he  has  seen  turkeys 
raised  from  the  eggs  of  the  wild  species  lose  their 
metallic  tints  and  become  spotted  with  white  in  the 
third  generation."  Again,  Mr.  Hewitt,  who  often 
reared  ducks  from  the  eggs  of  the  wild  bird,  and  who 
was  careful  that  there  should  be  no  crossing  with  do- 
mestic breeds,  "  found  that  he  could  not  breed  these 
wild  ducks  true  for  more  than  five  or  six  generations, 
as  they  proved  so  much  less  beautiful.  The  white  col- 
lar round  the  neck  of  the  mallard  became  much 
broader  and  more  irregular,  and  white  feathers  ap- 

,  ii.  p.  250. 


354  ACTION  OF  NATURAL  SELECTION 

peared  in  the  ducklings'  wings.  They  increased  also 
in  size  of  body;  their  legs  became  less  fine,  and  they 
lost  their  elegant  carriage.  Fresh  eggs  were  then  pro- 
cured from  wild  birds,  but  the  same  result  followed." 
Again,  Darwin*  records  that  "  according  to  Pallas, 
and  more  recently  according  to  Erman,  the  fat-tailed 
J£irghisian  sheep,  when  bred  for  a  few  generations  in 
Russia,  degenerate,  and  the  mass  of  fat  dwindles  away, 
the  scanty  and  bitter  herbage  of  the  steppes  seems  so 
essential  to  their  development."  The  fleece  of  sheep 
imported  from  Europe  to  the  West  Indies  is  much  af- 
fected, and  "  after  the  third  generation,  the  wool  dis- 
appears from  the  whole  body,  except  over  the  loins; 
and  the  animal  then  appears  like  a  goat  with  a  dirty 
door-mat  on  its  back.  A  similar  change  is  said  to  take 
place  on  the  West  Coast  of  Africa."  Another  in- 
stance of  the  effect  of  climate  on  sheep  is  recorded  by 
Brewer.f  Sheep  taken  from  southeastern  Ohio,  a  dis- 
trict noted  for  its  excellent  wool,  and  pastured  on  the 
alkaline  soil  of  a  certain  portion  of  Texas,  had  the 
texture  of  their  wool  much  altered,  and  its  reaction  to 
dyes  showed  obvious  differences.  Brewer  states  that 
"  the  change  in  the  character  of  the  wool  begins  imme- 
diately, but  is  more  marked  in  the  succeeding  fleeces 
than  in  the  first.  It  is  also  alleged  that  the  harshness 
increases  with  succeeding  generations,  and  that  the 
flocks  which  have  inhabited  such  regions  several  gener- 
ations produce  naturally  a  harsher  wool  than  did  their 
ancestors,  or  do  the  newcomers." 

The  deteriorating  effect  of   an  Indian   climate  on 

*Ibid.,i.  p.  102. 

t  Vide  Cope's  "  Factors  of  Organic  Evolution,"  p.  435. 


ON  VARIATIONS.  355 

dogs  is  well  known.  Hounds  seem  to  degenerate  most 
rapidly  of  all,  whilst  greyhounds  and  pointers  also  de- 
cline rapidly.  Darwin  *  was  informed  by  Dr.  Falconer 
that  bull-dogs  "  not  only  fall  off  after  two  or  three 
generations  in  pluck  and  ferocity,  but  lose  the  under- 
hung character  of  their  lower  jaws;  their  muzzles  be- 
come finer  and  their  bodies  lighter."  He  also  men- 
tions a  case  of  a  pair  of  setters,  born  in  India,  which 
perfectly  resembled  their  Scotch  parents.  Several 
litters  were  raised  from  them  in  Delhi,  but  none  of  the 
young  dogs  obtained  resembled  their  parents  in  size  or 
make,  their  nostrils  being  more  contracted,  their  noses 
more  pointed,  their  limbs  more  slender,  and  their  size 
inferior.  On  the  coast  of  Guinea,  "  dogs,  according  to 
Bosnian,  alter  strangely;  their  ears  grow  long  and  stiff 
like  those  of  foxes,  to  which  colour  they  also  incline, 
so  that  in  three  or  four  years  they  degenerate  into  very 
ugly  creatures ;  and  in  three  or  four  broods  their  bark- 
ing turns  into  a  howl." 

Darwin  considers  this  tendency  to  rapid  deteriora- 
tion in  European  dogs  may  be  largely  attributed  to  re- 
version. It  is  of  course  possible  that  this  may  be  the 
case,  but  it  seems  to  me  more  probable  that  it  is  due  to 
the  direct  and  cumulative  effects  of  changed  conditions 
of  life. 

The  cumulative  effect  of  conditions  of  life  is  ad- 
mitted, even  ]py  Weismann,  in  the  case  of  the  butterfly 
Polyommaius  phlcuas.  As  already  mentioned  in  Chap- 
ter VII.,  this  occurs  as  a  reddish  gold  variety  in  Ger- 
many and  other  countries  of  similar  latitude,  and  as  a 
much  darker  variety  in  more  southerly  countries,  as 
*Ibid.,\.  p.  39. 


356  ACTION  OF  NATUEAL  SELECTION 

Italy  and  Greece.  Though  these  forms  can  be  more  or 
less  transformed  into  each  other,  by  suitable  exposure 
of  the  pupae  to  warmth  or  cold,  yet  Weismann  found 
that  from  German  pupae  he  could  never  obtain  butter- 
flies so  dark  as  the  darkest  forms  of  the  southern 
variety,  whilst  from  Neapolitan  pupae  he  could  never 
get  them  so  light  as  the  ordinary  German  variety.* 
It  seemed  to  him,  therefore,  "  that  the  two  varieties 
may  have  originated  owing  to  a  gradual  cumulative  in- 
fluence of  the  climate,  the  slight  effects  of  one  summer 
or  winter  having  been  transmitted  and  added  to  from 
generation  to  generation."  Weismann  explains  this 
case  of  apparent  transmission  of  acquired  characters  by 
supposing  that  the  temperature  slightly  affects  the  de- 
terminants of  the  wing  scales  contained  in  the  germ- 
plasm,  as  well  as  more  markedly  influencing  the  deter- 
minants of  the  rudimentary  wings  in  the  chrysalis. 
Moreover  he  suggests  that  "  in  many  other  animals  and 
plants  influences  of  temperature  and  environment  may 
very  possibly  produce  permanent  hereditary  variations 
in  a  similar  manner." 

This  suggestion  of  Weismann's  contains  in  it,  it 
seems  to  me,  the  germ  of  an  idea  which  further  obser- 
vation and  experiment  may  prove  to  be  of  fundamental 
importance  in  evolution.  The  idea  itself  is  no  new 
one,  and  has  probably  occurred  independently  to  many 
writers.  As  far  as  I  am  aware,  it  was  first  suggested 
by  Galton,f  when  propounding  the  theory  of  heredity 
to  which  that  of  Weismann  bears  so  striking  a  resem- 

*"  Germ-Plasm,"  p.  399. 

fProc.   Roy.   Soc.,   xx.   p.   394,   1872;    Contemporary    Review, 
December,  1875;  Journ.  Anthropol.  Inst.,  1875,  p.  346. 


ON  VARIATIONS.  357 

blance.  Thus  he  concluded  that  we  are  almost  justified 
in  reserving  our  belief  that  the  body  cells  can  react 
on  the  sexual  elements,  i.  e.,  that  acquired  characters 
can  be  inherited;  but  he  himself  proposed  to  accept 
the  supposition  of  their  being  faintly  heritable.  More 
recently,  Cope*  has  embodied  the  idea  in  his  "  Theory 
of  Diplogenesis."  Thus  he  says,  "  Now,  since  these 
somatic  cells  develop  the  modifications  which  constitute 
evolution  in  their  subsequent  growth  into  organs,  there 
is  no  reason  why  the  reproductive  cells  which  experi- 
enced similar  influences  should  not  develop  similar 
characters,  so  soon  as  they  also  are  prepared  to  grow 
into  organs.  .  .  The  effects  of  use  and  disuse  are  two- 
fold, viz. :  the  effect  on  the  soma,  and  the  effect  on  the 
germ-plasm.  .  .  The  character  must  be  potentially  ac- 
quired by  the  germ-plasma,  as  well  as  actually  by  the 
soma."  However,  when  Cope  begins  to  briefly  expand 
his  theory,  he  seems  to  me  to  drift  into  improbable 
and  unverifiable  speculations.  Thus  he  imagines  that 
the  transmission  of  external  influences  is  primarily 
through  the  nervous  system — perhaps  through  the 
organisation  of  some  peculiar  mode  of  motion — and 
secondarily  through  nutrition. 

In  order  to  account  for  the  numerous  instances  of 
the  cumulative  effects  of  changed  conditions  of  life,  it 
seems,  therefore,  that  we  may  assume  with  consider- 
able probability  and  reason  that  the  germ-plasm  is  di- 
rectly affected  as  well  as  the  body  tissues.  These  ap- 
parent instances  of  the  inheritance  of  acquired  char- 
acters are  in  reality,  therefore,  nothing  of  the  kind,  but 
are  due  to  the  germ-plasm  reacting  to  change  of  en- 
*  Amer.  Naturalist,  xxiii.  p.  1058,  1889. 


358  ACTION  OF  NATURAL  SELECTION 

vironment  simultaneously  with  the  body  tissues.  As 
Weismann  points  out,  a  necessary  corollary  to  this  view 
is  "  the  assumption  of  material  determinants  which 
exist  in  the  germ-plasm  and  are  passed  on  from  one 
generation  to  another."  If  change  of  environment 
acts  cumulatively  on  the  fleece  of  the  sheep,  or  the 
structural  characters  of  a  dog,  it  follows  that  it  must 
in  each  of  the  first  few  generations  act  also  on  the  "  de- 
terminants "  in  the  germ-plasm  specifically  represent- 
ing such  specific  characters.  The  effect  produced  on 
such  determinants  in  the  first  generation  must  serve 
more  or  less  as  a  starting  point  for  the  environment  to 
work  upon  still  further  in  the  next  generation,  and 
so  on. 

Through  what  agency  is  the  environment  enabled  to 
act  on  the  germ-plasm?  To  me  the  only  conceivable 
one  is  a  chemical  influence,  through  products  of  metab- 
olism and  specific  internal  secretions.  We  have  seen 
in  a  previous  chapter  that  the  products  of  metabolism 
of  an  organism  may  exert  a  retarding  effect  on  its 
own  growth,  and  in  some  cases  a  stimulating  effect  on 
the  growth  of  other  organisms.  Physiological  re- 
search of  the  last  few  years  has  shown  that  most 
of  the  organs  and  tissues  of  the  body  have  specific 
internal  secretions,  which,  passing  into  the  general  cir- 
culation, may  exert  an  influence  of  vital  importance  on 
the  general  metabolism  of  the  organism.  Thus  extir- 
pation of  the  thyroid  gland  produces  symptoms  which 
in  many  animals  end  fatally,  but  which  may  be  dimin- 
ished or  suppressed  by  feeding  on  the  gland  substance, 
or  injection  of  extracts  of  it.  Extirpation  of  the 
suprarenal  glands  results  in  much  more  speedy  death, 


ON  VARIATIONS.  359 

and  here  again  the  injection  of  extracts  may  delay  the 
fatal  issue.  Extirpation  of  the  pancreas  causes  the 
production  of  severe  diabetes,  and  ultimately  death, 
but  such  an  effect  may  be  avoided  by  the  grafting  of  a 
portion  of  excised  gland  in  the  peritoneal  cavity  or  the 
tissues.  In  such  a  case  it  cannot,  of  course,  exercise 
its  digestive  function,  but  its  internal  secretion  pre- 
vents the  onset  of  the  fatal  diabetes.  Again,  extirpa- 
tion of  the  total  kidney  substance  of  a  dog  leads,  not 
to  a  diminished  secretion  of  urine,  but  to  a  largely  in- 
creased secretion,  accompanied  by  a  rapid  wasting  away 
which  soon  ends  fatally.  Hence  the  kidneys  may  pos- 
sess an  influence  on  the  metabolism  of  the  whole  body, 
as  well  as  their  obvious  secretory  function.  The 
spleen  appears  to  have  an  internal  secretion  which 
is  of  influence  in  setting  free  the  pancreatic  ferment. 
Finally,  extracts  of  various  nervous  tissues,  brain, 
spinal  cord,  and  sciatic  nerve,  have  been  found  when 
intravenously  injected  to  produce  a  distinct  fall  of 
blood  pressure,  whilst  those  of  the  pituitary  body  pro- 
duce a  marked  rise. 

Does  it  not  seem  distinctly  probable,  therefore,  that 
every  tissue  in  the  body  to  some  extent  affects  every 
other  tissue?  Each  may  have  its  own  specific  products 
of  metabolism,  and  perhaps  specific  internal  secre- 
tions, which,  passing  into  the  general  circulation,  may 
in  turn  stimulate  or  depress,  or  otherwise  affect, 
every  other  tissue  in  the  body.  Whenever  a  changed 
environment  acts  upon  the  organism,  therefore,  it  to 
some  extent  affects  the  normal  excretions  and  secre- 
tions of  some  or  all  of  the  various  tissues,  and  these 
react  not  only  on  the  tissues  themselves,  but  also  to  a 


360  ACTION  OF  NATURAL  SELECTION 

lesser  degree  upon  the  "  determinants  "  representing 
them  in  the  germ-plasm. 

It  should  be  mentioned  that  the  influence  of  somatic 
variations  on  the  germ-plasm  through  the  agency  of 
various  secretions  has  already  been  suggested  by  De- 
lage.*  Though  he  does  not  admit  Weismann's  doctrine 
of  determinants,  he  thinks  that  the  ovum  may  contain 
specific  substances  of  an  identical  nature  to  those  con- 
tained in  the  cells  of  the  principal  classes  of  tissues, 
such  as  the  nervous,  muscular,  and  perhaps  glandular. 
Conditions  of  life  such  as  climate  and  food,  which 
through  the  intermediation  of  the  blood  influence  the 
constituents  of  certain  of  the  body  tissue  cells,  will 
therefore  influence  the  same  substance  in  the  ovum,  or 
produce  hereditary  variations. 

The  hypothesis  of  specific  secretions  is  of  distinct 
help  in  accounting  for  certain  apparent  instances  of 
the  inherited  effects  of  use  and  disuse.  As  we  have 
seen  in  a  former  chapter,  Darwin  found  that  the  rela- 
tive size  of  the  brain  of  the  domestic  rabbit  has  con- 
siderably diminished.  Possibly  this  may  have  been 
the  result  of  more  ample  food,  and  of  artificial  selection 
of  individuals  with  large  bodies  and  small  heads,  and 
of  panmixia  (cessation  of  Natural  Selection),  but  it 
seems  almost  more  probable  that  it  is  due,  at  least  in 
part,  to  the  inherited  effects  of  disuse.  Thus  a  rabbit, 
when  kept  in  captivity,  would  need  to  use  its  brain  but 
little,  and  hence  the  excretions  and  secretions  of  the 
nervous  tissues  would  be  diminished.  The  "  determi- 
nants "  in  the  germ-plasm  corresponding  to  these 
would  be  less  stimulated  than  in  wild  rabbits,  and 
*  "  Heredite,"  pp.  806  to  812,  Paris,  1895. 


ON  VARIATIONS.  361 

hence  in  the  next  generation  the  development  of  the 
brain  (and  probably  the  other  nervous  tissues)  would 
take  place  somewhat  less  vigorously,  and  the  adult 
brain  be  in  consequence  somewhat  diminished  in  size. 
In  the  next  generation  the  diminution  would  be  greater 
still,  and  so  on. 

Again,  we  have  seen  that  in  man,  for  instance,  the 
degree  of  (hereditary)  pigmentation  of  the  skin  seems 
to  vary  closely  with  the  intensity  of  the  heat  and  light 
experienced.  It  is  possible  that  the  specific  excretory 
products  of  the  pigment  deposited  in  the  skin,  as  a 
direct  response  to  the  action  of  the  environment,  may 
stimulate  the  pigment  "  determinants  "  in  the  germ- 
plasm  to  increased  vigour,  so  that  in  the  next  genera- 
tion the  organism  will  tend  to  become  slightly  more 
pigmented  than  it  had  been  in  the  previous  one.  Sup- 
posing, on  the  other  hand,  the  pigment  cells  of  the  skin 
received  no  light  rays  whatsoever,  as  in  animals  which 
had  wandered  into  a  subterranean  cave,  their  metab- 
olism would  be  reduced  almost  to  nil,  and  so  the  pig- 
ment "  determinants "  in  the  germ-plasm  would 
diminish  in  vigour,  and  the  offspring  of  the  animals 
would  be  (at  birth)  somewhat  less  pigmented  than  they 
had  been  in  previous  generations. 

It  is  obvious  thajb  on  our  specific  secretion  hypothesis 
only  a  certain  class  of  acquired  characters  can  be  in 
any  degree  heritable;  only  those,  in  fact,  of  which  the 
corresponding  tissues  possess  a  specific  secretion  or  ex- 
cretion, capable  of  acting  specifically  on  the  "  deter- 
minants "  of  such  tissues  in  the  germ-plasm.  For  in- 
stance, the  blacksmith  cannot  transmit  his  brawny  arm 
in  any  degree  to  his  descendants,  as  it  is  scarcely  pos- 


362  ACTION  OF  NATURAL  SELECTION 

sible  that  the  arm  muscles  can  have  a  secretion  differ- 
ent from  that  of  the  other  muscles  of  the  body.  The 
greater  muscular  development  of  the  man  as  a  whole, 
however,  may  lead  to  the  production  of  slightly  more 
muscular  children  than  the  average. 

On  our  hypothesis,  the  heritableness  of  mutilations 
and  injuries  is  not  admissible.  It  is  almost  inconceiv- 
able that  each  spot  of  skin  on  the  body,  or  each  finger, 
should  have  a  specific  secretion,  and  that  an  injury  to 
it,  by  changing  its  secretion,  should  so  affect  the  germ- 
plasm  as  to  produce  a  similar  change  in  the  correspond- 
ing area  of  skin  or  the  finger  of  the  offspring.  How, 
then,  is  it  possible  to  account  for  the  various  apparent 
instances  of  inherited  injuries,  such  as  are  quoted  by 
Eimer,*  Cope,f  and  others  who  believe  in  the  transmis- 
sibility  of  such  characters?  There  certainly  seem  to 
be  a  small  number  of  thoroughly  well  authenticated 
cases,  but  the  number  is  so  small  that  we  may  perhaps 
attribute  them  to  mere  coincidence.  The  millions  of 
instances  of  injuries  which  show  no  trace  of  any  trans- 
mission provoke  no  remark,  as  it  is  only  what  we  are  led 
by  common  experience  to  expect.  Supposing,  on  the 
other  hand,  a  child  exhibits  any  birth  mark  or  de- 
formity bearing  some  similarity  to  an  injury  or  mutila- 
tion in  a  parent,  it  is  at  once  hailed  as  a  remarkable 
case  of  inheritance  of  an  acquired  character. 

There  are,  however,  certain  cases  of  the  apparent  in- 
heritance of  acquired  characters  which  require  more 
detailed  criticism.  These  are  the  well-known  experi- 
ments and  observations  of  Brown-Sequard  on  injuries 

*  "  Organic  Evolution/'  p.  173. 

f  "  Factors  of  Organic  Evolution,"  p.  431. 


ON  VARIATIONS.  363 

of  the  nervous  system  in  guinea-pigs.  As  Brown- 
Sequard  experimented  over  a  period  of  thirty  years  on 
thousands  of  guinea-pigs,  it  might  be  thought  that  we 
could  accept  his  results  as  absolutely  conclusive.  Yet 
a  repetition  of  some  of  his  experiments  by  Romanes 
and  by  Hill  seems  to  show  that  they  may  be  very 
largely  erroneous.  Thus,  like  Brown-Sequard,  Ro- 
manes *  found  that  some  of  the  progeny  of  parents  in 
which  an  injury  to  the  restiform  body  had  produced 
protrusion  of  the  eyeball,  showed  a  protrusion  likewise, 
though  this  was  less  marked,  and  always  affected  both 
eyes;  but  it  seemed  that  this  might  be  an  accidental 
occurrence,  in  that  normal  guinea-pigs  are  sometimes 
to  a  certain  extent  exophthalmic.  Again,  Romanes 
found  that  some  of  the  progeny  of  animals  in  which 
hsematoma  and  dry  gangrene  of  the  ears  had  super- 
vened after  injuring  the  restiform  body,  also  became 
affected.  However,  the  morbid  state  seemed  to  arise 
at  any  time  in  the  life  history  of  the  individual,  and  the 
process  not  only  affected  a  much  less  quantity  of  the 
ear,  but  also  a  different  part  of  it.  One  therefore 
might  imagine  it  to  be  due  to  mere  coincidence,  or  to 
transmitted  microbes;  but  Romanes  does  not  think  this 
can  be  the  case,  as,  on  the  one  hand,  he  has  never  seen 
the  peculiar  morbid  process  of  the  ears  in  other  guinea- 
pigs,  and,  on  the  other  hand,  he  was  unable  to  inoculate 
the  ears  of  healthy  animals  with  matter  from  the  ears 
of  mutilated  guinea-pigs. 

Romanes  repeated  Brown-Sequard's  experiments  on 
the  section  of  the  cervical  sympathetic  nerve,  but  he 
never  observed  in  their  progeny  any  change  in  the  shape 
*  "  Darwin  and  after  Darwin,"  vol.  ii.  p.  104  et  seq. 


364  ACTION  OF  NATURAL  SELECTION 

of  the  ear  or  partial  closure  of  the  eyelids.  Dr.  Leonard 
Hill*  has  also  repeated  them  with  some  thoroughness. 
The  operation  was  performed  on  six  guinea-pigs,  and 
these  animals  were  allowed  to  interbreed.  It  was 
again  performed  on  twelve  of  their  offspring,  and  these 
were  also  allowed  to  interbreed,  but  none  of  the  young 
of  either  the  first  or  the  second  generation  showed  any 
persistent  droop  of  the  eyelid.  Hill  found,  however, 
that  many  of  the  young  guinea-pigs  exhibited  a  partial 
closure  of  the  eye  for  some  time  after  birth,  but  this 
phenomenon  was  due  entirely  to  conjunctivitis,  the  re- 
sult of  dirt  getting  into  the  eyes.  It  affected  both  eyes 
equally  often,  and  when  it  terminated  the  droop  disap- 
peared also.  One  is  strongly  tempted  to  conclude  that 
the  partial  closure  of  the  eyelids  observed  by  Brown- 
Sequard  was  due  to  a  similar  cause,  and  was  no  more 
hereditary  than  in  Hill's  guinea-pigs.  Certain  of 
Brown-Sequard's  experiments  have,  however,  been  cor- 
roborated by  subsequent  observers,  and  must  therefore 
be  accepted.  Thus  he  found  that  animals  which  had 
been  rendered  epileptic  by  injury  to  the  spinal  cord,  or 
section  of  the  sciatic  nerve,  might  transmit  this  epilepsy 
to  their  offspring.  These  results  have  been  confirmed 
by  Obersteiner,f  and  Westphal  has  even  succeeded  in 
producing  epilepsy,  which  was  transmitted  to  the  off- 
spring, by  striking  guinea-pigs  on  the  head  with  a  ham- 
mer. It  has  been  suggested  by  "Weismann  that  the 
transmission  might  be  due  to  the  introduction  of  some 
microbe  into  the  operative  wound,  which  both  caused 
epilepsy  in  the  parent,  and,  by  invading  the  germ  cells, 

*  Proc.  Zool.  Soc.  1896,  p.  785. 

f  Oesterreiohiscbe  medicinische  Jahrbiicher,  1875,  p.  179. 


ON  VARIATIONS.  365 

produced  it  in  the  offspring  also.  This  could  not  have 
been  the  case  in  Westphal's  experiments,  however,  as 
in  them  no  wound  at  all  was  made. 

How,  then,  can  this  apparent  transmission  of  ac- 
quired characters  be  accounted  for?  Our  hypothesis 
of  internal  secretions  supplies  a  very  simple  explana- 
tion. Thus  the  secretions  from  the  brain  of  an  epi- 
leptic guinea-pig,  no  matter  how  this  epilepsy  had  been 
produced,  would  almost  certainly  be  abnormal.  Even 
supposing  that  they  were  without  effect  on  the  "  de- 
terminants "  of  the  nervous  tissues  in  the  germ-plasm, 
it  is  a  very  probable  supposition  that  they  might  so 
affect  the  growth  of  the  nervous  tissues  of  the  offspring, 
during  intra-uterine  development,  as  to  provoke  a 
similar  abnormal  condition  in  them. 

In  mammals  and  other  viviparous  animals,  it  is  prob- 
able that  changed  conditions  of  life  produce  part  of 
their  cumulative  action  during  the  period  in  which  the 
embryo  is  under  the  influence  of  the  maternal  fluids. 
It  is  of  course  possible  that  all  of  the  cumulative  effect 
is  then  produced,  though  in  such  a  case  we  should  have 
to  find  some  other  explanation  than  that  given  above  of 
the  cumulative  effects  noticed  in  oviparous  animals  as 
Polyommatus  phlceas.  In  the  case  of  the  gradual  de- 
generation of  the  pure  bred  dog  under  an  Indian 
climate,  for  instance,  the  environment  may  so  act  upon 
the  maternal  parent  as  to  produce  slight  changes  in  the 
body  tissues,  and  also  to  alter  the  character  of  the 
secretions  and  excretions.  These,  acting  on  the  off- 
spring during  their  embryonic  development — when, 
as  we  have  seen  in  a  previous  chapter,  the  tissues 
are  extraordinarily  sensitive  to  their  environment 


366  ACTION  OF  NATURAL  SELECTION 

— may  produce  more  obvious  degenerative  changes, 
which  will  of  course  continue  and  be  increased 
during  extra-uterine  growth.  This  second  genera- 
tion of  dogs,  besides  being  modified  in  external 
characters,  will  therefore  have  the  nature  of  their 
internal  secretions  more  altered  than  had  the  first 
generation.  These  changes  will  react  still  further  on 
their  offspring  during  intra-uterine  development,  and 
BO  on. 

Our  conclusions  as  to  the  reaction  of  the  germ-plasm 
to  the  external  conditions  of  environment  place  a  much 
higher  value  on  somatic  variations  as  a  factor  in  Evolu- 
tion than  that  accepted  by  Weismann  and  his  followers. 
It  is  for  this  reason  that  the  effects  of  environment  in 
the  production  of  variations  have  been  dealt  with  at 
such  length  in  the  preceding  chapters.  Every  obvious 
effect  produced  in  an  organism  by  the  direct  action  of 
the  environment,  may,  in  my  opinion,  be  accompanied 
by  a  more  or  less  corresponding,  though  much  slighter, 
effect  upon  the  determinants  in  the  germ-plasm,  and 
express  itself  in  the  next  generation  as  an  apparently 
cumulative  effect  of  the  changed  environment.  How 
often  this  possible  influence  on  the  germ-plasm  actually 
shows  itself,  and  what  may  be  the  numerical  measure 
of  its  extent,  can  only  be  determined  by  long  continued 
observation  and  experiment. 

As  we  shall  see  in  the  next  chapter,  somatic  varia- 
tions may  be  of  very  great  importance  in  evolution  by 
reason  of  their  adaptiveness  to  sudden  changes  of  en- 
vironment; but,  quite  apart  from  any  question  of  adap- 
tation, it  is  probable  that  they  may  be  of  value  in 
affording  Natural  Selection  a  better  chance  of  exert- 


ON  VARIATIONS. 


367 


ing  its  influence.     How  this  is  so,  is  best  explained  by 
means  of  a  diagram. 


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FIG.  29.  —  Effect  of  variable  environment  on  variability 
of  organisms. 


368  ACTION  OF  NATURAL  SELECTION 

The  upper  of  the  two  accompanying  figures  represents 
a  (roughly)  normal  curve  of  distribution  of  324  meas- 
urements (represented  by  dots  and  crosses).  As  is  in- 
dicated on  the  base  line,  these  vary  in  size  from  the 
general  mean  by  from  ±  1  to  ±10  per  cent.  Sup- 
posing that  the  larger  individuals  (the  crosses)  were 
better  adapted  to  their  environment,  and  were  being 
gradually  selected  by  the  agency  of  Natural  Selection, 
whilst  the  shorter  ones  were  gradually  being  weeded 
out,  then  it  is  probable  that  the  selective  process  would 
act  only  very  slowly,  as  the  range  of  variation  is  so 
slight.  Thus  differences  of  2  or  4  per  cent,  from  the 
average  are  so  small  as  to  be  almost  inappreciable, 
whilst  greater  differences,  as  of  7  or  8  per  cent.,  are 
exhibited  by  only  a  very  small  proportion  of  the  whole 
(only  12  out  of  the  324  measurements  being  7  per  cent, 
greater  than  the  average,  and  only  6  of  them  8  per 
cent,  greater).  Now  let  us  suppose  that  this  group  of 
324  individuals  is  acted  on  by  a  variable  environment, 
so  that  the  slight  range  of  blastogenic  variations  is  en- 
hanced by  the  superposition  of  somatic  modifications. 
Out  of  every  ten  organisms  of  any  particular  size,  let 
one  be  increased  by  2  per  cent.,  another  by  4  per  cent., 
another  by  6  per  cent.,  another  by  8  per  cent.,  and  an- 
other by  10  per  cent.,  owing  to  the  action  of  a  favour- 
able environment,  whilst  the  other  five  of  the  ten 
organisms  are  diminished  by  similar  amounts,  owing  to 
the  action  of  an  unfavourable  environment.  The  lower 
figure  shows  the  new  distribution  of  the  organisms  ac- 
cording to  their  altered  magnitudes.  For  instance,  of 
the  40  crosses  representing  individuals  1  per  cent, 
larger  than  the  average,  4  are  increased  by  2  per  cent., 


ON  VARIATIONS.  369 

and  so  are  placed  in  the  -p3  per  cent,  column, 
and  4  are  diminished  by  2  per  cent.,  and  so  are  placed 
in  the  —  1  per  cent,  column.  Four  more  are  placed 
in  the  +  5  per  cent,  column,  and  4  in  the  —  3  per 
cent,  column,  and  so  on.  A  similar  process  was  ap- 
plied to  all  the  other  measurements,  as  far  as  possible, 
the  6  individuals  7  per  cent,  larger  than  the  average 
being,  for  instance,  increased  and  diminished  by  ±  4, 
6,  and  8  per  cent.  The  curve  of  distribution  of  the 
324  individuals  now  takes  the  form  of  the  lower  figure. 
"We  see  that  it  is  much  more  flat-topped,  indicating  that 
the  range  of  variation  is  much  greater  than  before. 
This  is,  in  fact,  more  than  doubled,  the  arithmetical 
mean  error  being  increased  from  ±3.2  per  cent,  to 
±6.8  per  cent.  The  individuals  now  vary  in  size  by 
±17  per  cent.,  so  Natural  Selection  can  act  with  much 
greater  celerity  and  certainty  than  before.  Thus  no 
less  than  32  of  the  324  individuals  are  now  11  per  cent, 
or  more  larger  than  the  average,  and  so  offer  a  very  ap- 
preciable handle  for  Selection  to  work  upon.  The  dis- 
tribution of  the  dots  and  crosses  shows  us,  also,  that  all 
the  extremely  large  individuals  are  also  individuals 
which  were  larger  than  the  normal  before  the  variable 
environment  was  brought  to  bear  on  them.  Many  of 
the  larger  individuals  were  rendered  smaller  than  the 
average  by  the  action  of  an  unfavourable  environment, 
and  many  of  the  smaller  rendered  larger  by  a  favour- 
able environment — i.  e.,  there  has  been  a  good  deal  of 
mixing  of  the  individuals  as  originally  distributed — but 
the  fact  remains  that  the  extremely  large  individuals, 
which  Natural  Selection  would  be  especially  likely  to 
favour,  and  the  extremely  small  ones,  which  it  would  be 


370  ACTION  OF  NATURAL  SELECTION 

especially  likely  to  eliminate,  are  still  those  which  were 
originally,  as  the  result  of  blastogenic  variation,  re- 
spectively larger  and  smaller  than  the  average.  The 
selected  individuals  are  therefore  not  only  larger  in 
themselves,  but  in  that  their  "  largeness  "  is  to  some 
extent  a  blastogenic  variation,  their  offspring  will  also 
be,  on  an  average,  larger  than  the  normal. 

It  is  not  intended  to  imply  that  increased  variability 
is  by  any  means  always  an  advantage.  In  a  stable 
form,  upon  which  Selection  is  acting  but  little,  it  might 
be  a  distinct  disadvantage,  as  the  more  variable  indi- 
viduals might  be  less  adapted  to  their  environment 
than  the  less  variable.  In  the  case  of  the  sparrow,  for 
instance,  we  saw  that  Bumpus  found  that  the  extreme 
individuals  in  either  direction  tended  to  be  weeded  out, 
though  there  was  a  much  greater  elimination  of  the  ex- 
treme individuals  in  one  direction  than  of  those  in  the 
other.  What  is  true  for  one  form,  however,  is  by  no 
means  necessarily  true  for  another,  and  in  a  rapidly 
evolving  organism,  such  as  the  pale-coloured  mouse 
found  on  the  sand-banks  off  Dublin,  it  is  probable  that 
the  eliminated  individuals  would  be  chiefly  confined  to 
the  darkest  specimens,  and  include  but  few  of  the  palest 
ones. 


CHAPTER  XII. 
ADAPTIVE    VAKIATIONS. 

Adaptability  a  fundamental  property  of  protoplasm— Instances  of 
adaptive  variation  in  plants — Acclimatisation  of  Protozoa  to  high 
temperature,  to  poisons,  to  mechanical  stimuli,  to  saline  solutions 
—Acclimatisation  of  fresh-water  Mollusca  to  salt  water,  and  of 
various  marine  animals  to  fresh  water — Acclimatisation  of  Mam- 
mals to  vegetable  poisons,  and  to  toxins— Sum  total  of  somatic 
variations  always  in  direction  of  adaptation — Somatic  varia- 
tions of  importance  in  evolution,  but  they  can  effect  little  without 
Natural  .Selection— Germinal  Selection. 

THE  question  of  the  definiteness  or  indefiniteness  of 
variations  has  been  frequently  and  hotly  debated,  but 
there  has  been  a  singular  absence  of  exact  definitions 
of  the  views  actually  held  by  the  supporters  of  the  rival 
theories.  Had  such  definitions  been  forthcoming,  I 
doubt  if  any  fundamental  differences  of  opinion  would 
have  been  found  to  exist  at  all.  Take,  for  instance, 
Darwin's  definition  of  definite  variations,  viz. :  "  The 
effects  of  (conditions  of  life).  .  .  may  be  considered 
as  definite  when  all  or  nearly  all  the  offspring  of 
individuals,  exposed  to  certain  conditions  during 
several  generations,  are  modified  in  the  same  man- 
ner." *  There  is  surely  nothing  in  this  definition 
which  would  not  be  generally  admitted.  As  has 
been  shown  at  some  length  in  several  of  the  preced- 
ing chapters,  change  in  one  or  many  of  the  conditions 
*  "  Origin  of  Species,"  p.  6. 

371 


372  ADAPTIVE  VARIATIONS. 

of  life  may  lead  to  very  considerable  changes  in 
the  form  and  structure  of  all  or  most  of  the  organisms 
exposed,  even  in  one  generation.  Hence,  if  Darwin's 
definition  be  accepted  as  it  stands,  we  are  compelled 
to  admit  that  variations  may  be  definite.  Suppos- 
ing, however,  it  be  taken  to  imply  the  cumulative, 
and  so  hereditary,  action  of  conditions  of  life  acting  for 
several  generations,  then  those  who  refuse  to  admit  the 
validity  of  the  instances  of  such  cumulative  action  ad- 
duced in  the  last  chapter  might  also  refuse  to  admit 
the  existence  of  definite  variations.  But  assuming  the 
former  interpretation  as  the  correct  one,  are  we  to 
agree  with  lienslow  *  that  "in  nature  variations  are 
always  definite,"  or  are  we  to  follow  Darwin  in  believ- 
ing that  variations  are,  as  a  rule,  indefinite,  and  only 
exceptionally  definite  ?  Here,  it  seems  to  me,  we  are  in 
want  of  more  exact  definitions.  Probably  Henslow 
would  admit  that  the  variations  in  the  number  of  car- 
pels in  the  common  daisy,  or  of  veins  in  the  leaf  of  the 
beech  tree,  or  of  stigmatic  bands  on  the  seed  capsules 
of  the  poppy,  are  governed  by  the  laws  of  chance,  or  if 
he  did  not,  how  could  he  account  for  the  fact  that  the 
frequencies  of  distribution  of  the  respective  numbers 
are  in  accordance  with  the  Law  of  Error?  Clearly,  in 
such  cases  the  variations  must  be  indefinite.  Suppos- 
ing, however,  that  the  distribution  of  the  variations  in 
the  length  of  the  leaves  of  a  plant  grown  upon  land 
occur  according  to  the  laws  of  chance,  whilst  that  of  the 
leaves  of  the  same  species  of  plant  grown  in  water  also 
follows  these  laws,  but  supposing  also,  that  the  average 
length  of  the  aquatic  plant  leaves  is  considerably 
*"  Original  of  Plant  Structures,"  p.  ix. 


ADAPTIVE  VARIATIONS.  373 

greater  than  that  of  the  land  plant  leaves,  then  obvi- 
ously we  should  have  here  a  case  of  both  definite  varia- 
tion and  indefinite  variation.  The  leaves  of  the  aquatic 
plant  would  have  varied  in  the  direction  of  greater 
length,  or  would  have  varied  definitely  in  adaptation  to 
their  new  environment,  but  the  distribution  of  their 
variations  about  their  mean  would  still  be  in  accord- 
ance with  the  laws  of  chance,  or  would  be  indefinite. 

The  term  "  definite,"  as  applied  to  variations,  seems 
to  be  generally  regarded  as  more  or  less  synonymous 
with  "  adaptive."  Thus  Lloyd  Morgan  *  defines  defi- 
nite or  determinate  variations  as  "  variations  along 
special  or  particular  lines  of  adaptation,"  while  Hen- 
slow  f  says  "  Definite  variations  are  always  in  the  direc- 
tion of  adaptation  to  the  environment  itself."  Hence 
it  seems  to  me  that  the  discussion  of  the  definiteness  or 
indefiniteness  of  variations  may,  for  practical  purposes, 
be  narrowed  down  to  the  following  questions:  (1) 
Have  conditions  of  life  an  appreciable  influence  on 
organisms,  and  if  so,  (2)  Is  this  influence  in  any  case 
cumulative,  i.  e.,  partly  inherited,  and  (3)  How  far  are 
the  effects  produced  adaptive?  The  first  two  questions 
I  have  already  endeavoured  to  answer  in  the  preceding 
chapters.  The  third  we  will  now  proceed  briefly  to 
inquire  into. 

As  far  as  the  limited  number  of  observations  avail- 
able can  show,  adaptability  would  seem  to  be  a  funda- 
mental property  of  protoplasm.  Whenever  an  organ- 
ism is  exposed  to  changed  conditions  of  life,  then  it  is 
found  that  the  original  want  of  adaptation  becomes 
gradually  and  progressively  diminished  with  increase 

*  "  Habit  and  Instinct,"  p.  311.  f  Ibid.,  p.  viii. 


374  ADAPTIVE  VARIATIONS. 

in  the  duration  of  the  exposure.  In  most  animals,  the 
change  in  the  direction  of  adaptation  is  slight,  but  it  is 
probably  always  there,  if  only  it  be  carefully  looked 
for.  In  plants  it  is,  as  a  rule,  greater,  and  may  be  ob- 
vious to  the  most  cursory  observation.  Instances  of 
it  have  already  been  described  at  some  length  in  pre- 
ceding chapters,  and  hence  it  is  unnecessary  to  do  more 
than  briefly  recall  these  here.  We  saw  that  Karsten 
found  that  a  kidney  bean  reared  in  the  dark  for  a 
month  or  two  weighed  20  per  cent,  more  than  one 
reared  in  the  light,  yet,  owing  to  the  absence  of  the 
stimulus  of  light,  its  leaves  did  not  weigh  a  fifth  as 
much.  Lothelier  found  that  plants  such  as  Berberis 
vulgaris  bore  non-spinescent  leaves  in  a  moist  atmos- 
phere, but  spines  and  spines  alone  in  a  perfectly  dry 
one.  Costantin  found  that  he  could  change  the  form 
of  Hippuris  at  will,  by  growing  the  aquatic  form  of  the 
plant  on  land,  and  the  terrestrial  form  in  water.  All 
the  leaves  produced  under  water  were  long,  undulated, 
and  delicate,  whilst  those  in  air  were  short,  erect,  and 
firm.  Costantin,  and  also  Godron,  obtained  very  simi- 
lar results  by  growing  other  aquatic  plants  on  land,  and 
terrestrial  ones  in  water,  the  change  being  always  in 
the  direction  of  adaptation  to  the  new  surroundings. 
Again,  Lesage  found  that  by  watering  various  plants 
with  water  containing  salt  they  developed  characters 
similar  to  those  exhibited  by  maritime  plants,  viz.,  in- 
creased thickness  of  leaves,  larger  and  more  numer- 
ous palisade  cells,  and  diminution  of. the  intercellular 
spaces  and  of  the  chlorophyll.  Bonnier  found  that 
plants  of  Teucrium  scorodonia,  when  grown  at  a  high 
situation  in  the  Pyrenees,  exhibited  features  character- 


ADAPTIVE  VARIATIONS.  375 

istic  of  alpine  plants,  viz. :  very  short  aerial  stems,  with 
hairy  and  dark  green  leaves,  and  compact  inflorescence. 
Seeds  gathered  from  these  plants  and  sown  in  Paris 
after  three  years  produced  elongated  stems,  with  less 
hairy  and  brighter  green  leaves,  or  plants  very  similar 
to  those  from  seeds  obtained  in  the  neighbourhood  of 
Paris. 

In  addition  to  changes  of  climate  and  soil,  plants  can 
adapt  themselves  also  to  mechanical  stresses  and 
strains.  Thus  Ray  *  sowed  a  mould  (Sterigmatocystis) 
in  two  vessels,  one  of  which  was  fixed,  and  the  other 
subjected  for  two  months  to  a  rapid  oscillatory  move- 
ment. Instead  of  a  thick  feltwork  of  mycelium,  this 
latter  vessel  contained  small,  perfectly  spherical,  elas- 
tic masses  consisting  of  entangled  filaments.  The  sup-  , 
porting  tissues  of  the  plant  were  strengthened  in  re- 
sponse to  the  violent  mechanical  strains,  the  mem-  [j- 
branes  being  twice  or  three  times  as  thick,  and  the  fila- 
ments having  many  more  partition  walls.  Again,  R. 
Hegler  f  found  that  "  the  hypocotyl  of  a  seedling  sun- 
flower, which  would  have  been  ruptured  by  a  weight  of 
160  gm.,  bore  a  weight  of  250  gm.  after  having  been 
subjected  for  two  days  to  a  strain  of  a  weight  of  150 
gm.  The  weight  was  subsequently  increased  to  400 
gm.  without  injury.  .  .  Leaf  stalks  of  Helleborus 
niger,  which  broke  with  a  weight  of  400  gm.,  were  able 
to  resist  one  of  35  kgm.  after  having  been  subjected  to 
a  strain  for  about  five  days."  Thus  protoplasm  has  the 
power  of  responding  to,  and  counteracting  the  action 

*C.  R.  Acad.  8ci.f  cxxiii.  p.  907. 

fBer.  Verkandl.  K.  Sachs.  Gesell.  Wiss.,  v.  p.  638,  1892  (quoted 
from  Henslow's  "  Origin  of  Plant  Structures,"  p.  204). 


376  ADAPTIVE  VARIATIONS. 

of,  external  mechanical  forces  by  the  formation  of  sup- 
portive tissues.  It  is  by  reason  of  this  power  that 
plants  grow  vertically  upwards  in  opposition  to  the 
force  of  gravity. 

Some  of  the  most  remarkable  instances  of  adaptation 
in  plants  are  those  relating  to  the  interchange  between 
roots  and  stems.  In  many  cases,  at  least,  it  would 
seem  that  when  a  subterranean  root  becomes  aerial,  its 
characters  tend  to  approach  to  those  of  a  stem,  whilst 
a  normally  aerial  stem,  grown  underground,  develops 
the  characters  of  a  root.  For  example,  "  An  old  acacia 
with  a  decaying  trunk  sent  down  an  aerial  root  from 
the  living  part,  about  six  feet  from  the  ground.  When 
it  had  been  rooted  in  the  soil  for  some  time,  it  became 
detached  by  the  wind;  the  root  then  became  a  '  stem,' 
the  upper  part  putting  out  foliage."  *  Again,  Dr. 
Lindley  records  that  "  a  young  willow  tree  had  its 
crown  bent  down  to  the  ground;  this  was  covered  with 
earth,  and  soon  emitted  an  abundance  of  roots.  The 
true  roots  were  then  removed  from  the  soil,  and  the 
stem  inverted.  The  roots  now  became  branches  and 
emitted  an  abundance  of  buds,  and  the  tree  ever  after- 
wards grew  upside  down."  Accompanying  such 
changes  of  function  are  found  corresponding  changes 
of  histological  structure.  Costantin  t  determined  the 
effect  of  growing  stems  of  brambles  underground,  and 
he  found  that  the  number  and  volume  of  the  cortical 
cells  increased,  the  collenchyma  disappeared,  the  liber 
fibres  diminished  or  disappeared,  and  starch  could  be 
formed  and  stored  up  in  the  parenchymatous  tissues. 

*  Quoted  from  Henslow,  ibid.,  p.  179. 

f  Bull,  de  la  Soc.  Bot.  de  Fr.,  p.  230,  1883. 


ADAPTIVE  VAEIATIONS.  377 

Moreover  these  modifications  were  uniform,  affected 
all  the  tissues,  and  were  rapidly  produced,  a  week  or 
two  sufficing. 

There  is  no  evidence,  as  far  as  I  am  aware,  to  show 
how  widespread  is  this  phenomenon  of  interchange  be- 
tween roots  and  stems,  and  hence  one  cannot  accept  it 
as  a  generalised  property  of  plants.  In  any  case  one 
must  bear  in  mind  that  it  may  not,  after  all,  be  a  case 
of  direct  adaptation  to  surroundings  in  the  ordinary 
acceptation  of  the  term,  but  may  be  the  calling  up,  in 
response  to  one  of  two  stimuli,  of  one  of  two  groups  of 
characters  long  since  acquired  by  the  plant  protoplasm. 

A  case  of  adaptation  which  appeared  to  be  to  some 
extent  hereditary  has  recently  been  recorded  by 
Errera.*  Conidia  of  the  mould  Aspergillus  niger  were 
cultivated  by  Dr.  Hunger  for  two  generations  in  Rau- 
lin's  nutritive  solution,  to  which  6  per  cent,  of  common 
salt  had  been  added,  and  when  placed  in  a  similar  salt 
solution  they  were  found  to  produce  spores  in  3f  days. 
Conidia  which  had  been  cultivated  in  the  salt  Raulin 
solution  for  only  one  generation  took  4  days  to  produce 
spores,  however,  whilst  those  which  had  been  culti- 
vated in  Raulin  solution  containing  no  additional  salt 
took  5  days.  On  the  other  hand,  when  some  of  the 
conidia  cultivated  under  the  three  sets  of  conditions 
were  placed  in  ordinary  Raulin  solution,  those  kept  two 
generations  in  salt  solution  showed  only  slight  sporifi- 
cation  in  5  days,  those  kept  one  generation  showed 
more  marked  sporification,  whilst  those  kept  through- 
out in  ordinary  Raulin  solution  spored  in  4  days. 
Spores  from  these  three  last  cultures  in  normal  Raulin 
*Bull.  Acad.  Roy.  Beligque,  p.  81,  1899. 


378  ADAPTIVE  VAEIATIONS. 

solution  were  then  sowed  in  a  solution  to  which  18.4 
per  cent,  of  salt  had  been  added.  After  5  days  the 
original  normal  Raulin  culture  showed  no  germination; 
that  originally  kept  one  generation  in  salt  Raulin  solu- 
tion showed  slight  germination,  and  that  originally  kept 
two  generations  distinct  germination.  Thus  the  adap- 
tation to  a  concentrated  salt  solution  was  not  entirely 
lost  even  after  rearing  in  a  normal  medium,  or  was  in 
some  degree  inherited,  especially  in  the  case  of  the 
greater  degree  of  adaptation  produced  by  the  growth 
of  two  generations  in  salt  solution.  Doubtless  this 
"  inheritance  of  acquired  characters  "  was  due  to  the 
salt  solution  influencing  the  germ  cells  at  the  same 
time  as  the  body  cells.  The  same  explanation  may  be 
used  to  account  for  the  somewhat  similar  results  ob- 
tained by  Ray  *  with  Sterigmatocystis  alba.  Oonidia 
of  this  mould  were  sown  in  a  solution  of  dextrose,  the 
development  taking  place  but  slowly,  owing  to  the  want 
of  adaptation  to  the  new  environment.  On  continuing 
the  culture  in  the  sugar  solution,  however,  the  rate  of 
development  gradually  increased  from  generation  to 
generation,  till,  finally,  the  sixth  generation  showed  a 
more  abundant  development  after  8  days  than  the  first 
one  had  after  15  days.  The  morphological  characters 
were  progressively  modified  in  addition,  so  that  the 
mould  came  finally  to  resemble  a  penicillium. 

In  the  members  of  the  Animal  Kingdom,  the  power 
of  adaptation  is,  as  a  rule,  far  less  marked  than  in  those 
of  the  Vegetable  Kingdom,  but  probably  it  is  present 
to  a  greater  or  less  extent  in  all  organisms,  from  the 
lowest  to  the  highest.  In  certain  Flagellata,  for  in- 
*Rev.  Gen.  de  Bot.,  ix.  p.  193,  1897. 


ADAPTIVE  VARIATIONS. 


379 


stance,  Dallinger  *  has  demonstrated  a  most  remarkable 
and  extreme  adaptability  to  high  temperature.  Start- 
ing at  15.6°  C.,  he  gradually  raised  the  temperature  of 
the  water  containing  these  monads  up  to  70.0°  C.,  when 
the  experiment  was  ended  by  an  accident.  The  exact 
times  in  the  course  of  the  experiment  at  which  a  given 
temperature  was  reached  are  not  mentioned,  but  from 
the  description  afforded  they  are  gathered  to  be 
roughly  those  given  in  the  accompanying  table  : 


Original  temperature, 
After    4  months, 

15.6°  C. 
21.1 

A1 

< 

fter  36  mo 
39 

nths, 

< 

34.4" 
38.9 

6 

22.8 

< 

41 

i 

41.7 

1       8 

23.3 

« 

48 

« 

68.3 

'     15 

25.6 

• 

60 

i 

58.9 

'     23 

26.7 

1 

61 

• 

61.1 

1     32 

33.9 

(Several  months  more) 

70.0 

From  this  we  see  that  the  experiment,  as  far  as  it  was 
carried,  lasted  about  six  years.  The  raising  of  the 
temperature  was  not  by  any  means  even,  the  organisms 
frequently  reaching  stages  at  which  for  months  at  a 
time  an  increase  of  temperature  of  half  a  degree  or  less 
was  immediately  followed  by  adverse  effects,  and  in 
some  instances  by  the  death  of  many  of  the  organisms. 
For  instance,  when  a  temperature  of  25.6°  had  been 
reached,  it  was  found  that  for  a  space  of  five  months 
the  temperature  could  not  be  raised  even  .3  of  a  degree 
without  a  distinctly  evil  effect  being  produced.  In 
fact,  it  was  found  that  the  progress  of  acclimatisation 
at  lower  temperatures  was,  as  a  rule,  much  slower  than 
at  high  ones.  Thus,  within  a  space  of  seven  months,  it 

*  Journ.  Roy.  Microsc.  Soc.,  vol.  vii.  p.  191,  1887. 


380  ADAPTIVE  VARIATIONS. 

was  found  possible  to  raise  the  temperature  of  the  or- 
ganisms from  41.7°  to  58.3°.  Also  to  raise  the  tem- 
perature from  61.1°  to  70.0°  took  only  a  few  months 
(number  not  stated).  As  to  the  absolute  upper  limit 
of  temperature  these  infusoria  can  withstand,  it  is  of 
course  impossible  to  judge,  but  there  seems  no  reason 
to  suppose  that  it  might  not  be  considerably  higher 
than  that  reached  by  Dallinger.  A  striking  proof  of 
the  altered  condition  of  the  organisms  was  furnished 
by  the  fact  that  some  of  those  acclimatised  to  70.0°, 
died  off  when  placed  in  a  suitable  nutritive  solution  at 
15.6°. 

This  acclimatisation  was  probably  for  the  most  part 
a  direct  adaptation  of  the  protoplasm  to  its  new  en- 
vironment, but  it  must  also  have  been  in  part  due 
to  natural  selection.  Dallinger  noticed  on  more  than 
one  occasion  that  a  good  many  of  the  organisms  were 
killed  off,  and  these  would  doubtless  have  been  the  less 
adaptable  ones,  the  more  adaptable  surviving.  Still, 
as  far  as  one  can  judge  from  the  brief  account  given, 
the  temperature  was  often  raised  over  considerable  in- 
tervals without  any  such  fatalities. 

Dallinger's  results,  in  addition  to  their  intrinsic 
value,  are  of  great  interest  in  that  they  enable  us  to 
account  for  the  presence  of  various  Protophyta,  such  as 
Oscillatorice  and  Nostocacece  in  hot  springs.  The  tem- 
perature of  many  of  these  springs  is  considerably  above 
60.0°  C.,  and  that  of  the  California  geysers,  in  which 
Nostocacecz  (possibly  Protococcus)  are  found,  reaches 
the  remarkable  temperature  of  93°.  Certain  metazoa, 
also,  are  stated  to  live  at  temperatures  considerably 
above  45°,  or  temperatures  which  prove  fatal  to  their 


ADAPTIVE  VARIATIONS.  381 

allies.  All  such  instances  as  these  *  are  probably  due 
to  gradual  acclimatisation,  accompanied  by  a  variable 
amount  of  selective  destruction  of  the  less  adaptable 
organisms. 

Other  observations  on  acclimatisation  among  Pro- 
tozoa have  been  made  by  Davenport  and  Neal.f  The 
acclimatisation  of  Stentor  cceruleus  to  weak  corrosive 
sublimate  solution  was  tested.  Stentors  kept  for  two 
days  in  .00005  per  cent,  solution  were  found,  on  immer- 
sion in  .001  per  cent,  solution  of  sublimate,  to  be  killed 
off  after  (on  an  average)  304  seconds'  exposure.  Sten- 
tors kept  in  pure  water,  on  the  other  hand,  were  killed 
after  only  83  seconds7  immersion.  Similar  results  were 
obtained  in  other  experiments,  it  appearing  that  within 
certain  limits  the  resistance  period  varied  directly  with 
the  strength  of  the  solution  in  which  the  protoplasm 
had  been  cultivated.  If,  however,  the  culture  solution 
were  too  strong,  (above  .0001  per  cent.),  the  organism 
became  so  weakened  that  it  was  less  resistant  to  the 
killing  solution  than  those  reared  in  pure  water.  As 
no  deaths  occurred  in  the  culture  solutions,  the  adapta- 
tion must  have  been  a  direct  one,  and  in  no  way  de- 
pendent on  natural  selection. 

Stentor  can  also  become  acclimatised  to  mechanical 
stimuli,  for  Castle  $  has  observed  a  colony  of  Stentors 
in  an  aquarium  being  constantly  struck  by  Tubifex 
moving  backwards  and  forwards,  and  yet  showing  no 
contraction  as  they  usually  do  when  struck. 

*For  a  detailed  account  see  a  paper  by  Davenport  and  Castle, 
Arch,  f .  Entwick.  d.  Organismen,  Bd.  ii.  p.  227,  1895. 
t  Arch,  f .  Entwick.  d.  Organismen,  Bd.  ii.  p.  564,  1896. 
\  Vide  Davenport's  "  Experimental  Morphology,"  p.  109. 


382  ADAPTIVE  VARIATIONS. 

Upon  acclimatisation  to  saline  solutions  a  consider- 
able number  of  observations  has  been  made,  especially 
in  the  case  of  Protozoa.  As  long  ago  as  1869  Czerny  * 
experimented  on  amoebae,  and  found  that  by  the  very 
gradual  addition  of  salt,  he  could  acclimatise  them  to  a 
4  per  cent,  solution.  With  the  unacclimatised  organ- 
isms, the  sudden  addition  of  .33  per  cent,  of  salt  had  in 
many  cases  a  fatal  effect,  though  some  were  able  to 
stand  even  a  1  per  cent,  solution.  None  could  resist  a 
2  per  cent,  solution,  however.  More  recently  Massart  f 
has  made  quantitative  determinations  of  the  acclimati- 
sation of  certain  ciliated  infusoria  to  solutions  of  potas- 
sium nitrate.  Unacclimatised  cysts  of  Vorticella  nebu- 
lifera  first  began  to  show  plasmolysis  when  the  strength 
of  the  potassium  nitrate  solution  in  which  they  were 
placed  amounted  to  1.2  per  cent.  On  the  other  hand, 
cysts  previously  kept  22  hours  in  a  1.8  per  cent,  solution 
did  not  show  any  plasmolysis  until  the  concentration  was 
raised  to  2.5  per  cent.  Observations  on  Colpoda  cucul- 
lus  gave  similar  results.  The  degree  of  effect  produced 
by  a  .8  per  cent,  solution  in  unacclimatised  organisms 
required  a  2.5  per  cent,  solution  in  organisms  previ- 
ously kept  22  hours  in  1.8  per  cent,  solution.  The 
capacity  for  acclimatisation  varies  greatly  in  different 
organisms,  for  Bichter  $  succeeded  in  acclimatising 
Tetraspora  to  16  per  cent,  sodium  chloride  solution, 
whilst  Spirogyra,  similarly  treated,  was  unable  to  resist 
even  a  .5  per  cent,  solution. 

The  acclimatisation  of  certain  of  the  metazoa  to 

*  Arch.  f.  mik.  Anat.,  Bd.  v.  p.  158,  1869. 
t  Arch,  de  Biol.,  ix.  p.  515,  1899. 
t  Flora,  1.  p.  4,  1892. 


ADAPTIVE  VARIATIONS.  383 

changes  of  salinity  appears  to  have  been  first  studied 
by  Beudant  *  more  than  eighty  years  ago.  He  placed 
a  number  of  fresh  water  molluscs,  such  as  Lymnaa, 
Planorbis,  Physa,  Ancylus,  and  Paludina  in  a  vessel  of 
water,  and  added  a  small  quantity  of  salt  every  day. 
After  a  few  months  the  water  contained  4  per  cent,  of 
salt,  and  170  of  the  original  400  molluscs  were  still  sur- 
viving. Of  another  400  kept  under  otherwise  similar 
conditions  in  fresh  water,  184  were  surviving.  All 
species  are  not  equally  adaptable,  however,  as  Unio  and 
Anodonta,  though  they  throve  well  in  fresh  water,  all 
died  in  salt.  Beudant  also  performed  the  converse  ex- 
periment of  acclimatising  marine  molluscs  to  fresh 
water.  He  made  observations  on  38  different  species 
of  the  genera  Haliotis,  Cerithium,  Buccinum,  Tellina, 
Venus,  Ostrea,  Pecten,  and  Mytilus.  He  added  fresh 
water  every  day,  so  that  after  five  months  the  animals 
came  to  live  in  absolutely  fresh  water.  Out  of  the  38 
species  experimented  with,  20  withstood  the  change 
perfectly  well.  The  experiment  was  started  with  610 
individuals,  and  of  these  375  survived.  Of  a  similar 
number  kept  for  the  same  length  of  time  in  normal  sea 
water,  401  survived,  or  only  4.2  per  cent.  more.  How- 
ever, all  the  other  18  species  experimented  with  died 
during  the  course  of  the  experiment.  In  still  another 
series  of  experiments,  Beudant  succeeded  in  acclimatis- 
ing marine  molluscs  to  a  solution  containing  no  less  than 
31  per  cent,  of  salts.  These  consisted  chiefly  of  sodium 
chloride,  but  contained  also  calcium  and  magnesium 
chlorides. 

Numerous   observations   on   the    acclimatisation    of 
*  Journal  de  Phys.,  Ixxxiii.  p.  268,  1816. 


384 


ADAPTIVE  VAKIATIONS. 


other  organisms  such  as  Myxomycetes,  Actinos- 
pherium,  Crustacea,  and  tadpoles  have  been  made  by 
other  observers,*  but  it  is  unnecessary  to  mention  more 
than  a  single  experiment,  one  made  by  De  Yarigny  t 
upon  a  number  of  different  species  of  marine  animals. 
Some  Carcinus  mcenas,  Pagurus  Prideauxii,  Dromia 
vulgaris,  Anthea  cereus,  Sagartia  parasitica,  Portunus 
puber,  Doris  tuberculata,  Venus,  Actinia  mesembryan- 
themum,  and  HolotJiuria  tubulosa  were  placed  in  an 
aquarium  supplied  with  a  constant  flow  of  water.  This 
water  was  gradually  diluted  more  and  more  with  fresh 
water,  with  the  following  results : 


ft,  . 

fc  WC3 

& 

|nH 

o  W!H 

ANIMALS  KILLED. 

H  H 

ANIMALS   KILLED. 

•"1  H 

g 

|K> 

ft 

gc> 

1 

22.2 

nil 

22 

68.7 

3 
6 
11 

33.3 

44.4 

nil 
1  C.  mcenas,  1  Pagurus 
1  C.  mcenas,  1  Dromia 

25 
29 

77.8 

1  Portunus,  3  Anthea 
2  Doris  and  2  Venus 

13 

55.6 

and  rest  of  Pagurus 

32 
35 

88.9 

1  Portunus,  1  Anthea 

17 

3    Sagartia,    2    Holo- 

thuria. 

On  the  38th  day,  when  the  experiment  was  ended, 
there  were  still  living  all  of  the  original  eight  Actinia 
mesembryanthemum  and  one  Carcinus  mcenas.  De 
Yarigny  suggests  that  the  greater  resistance  of  A. 
mesenibryantJiemum  is  probably  connected  with  the  fact 
that  these  organisms  are  attached  to  rocks  near  the  sea 

*For  literature  vide  Davenport's  "Experimental  Morphology," 
p.  86. 
tCentralb.  f.  Physiol.,  i.  p.  566. 


ADAPTIVE  VAEIATIONS.  385 

surface,  and  so  are  frequently  uncovered.  They  must 
therefore  be  exposed  to  sea-water  diluted  by  rivers,  and 
to  rain  water.  The  less  resistant  Anthea  is  found  fur- 
ther below  the  surface,  however,  whereas  the  still 
less  resistant  Sagartia  lives  in  water  several  metres 
deep. 

Observations  on  acclimatisation  to  saline  solutions 
are,  perhaps,  less  important  and  less  interesting  than 
those  on  acclimatisation  to  other  conditions,  in  that, 
within  certain  limits,  the  phenomenon  is  probably  a 
purely  physical  one,  dependent  on  differences  of  osmo- 
sis, and  the  pressures  and  strains  thereby  set  up. 
There  is  little  doubt  that  if  sufficient  care  and  time  be 
employed,  any  marine  organism  could  be  acclimatised 
to  fresh  water,  and  any  fresh  water  form  to  salt  water, 
or  solutions  of  even  greater  density.  If  it  be  remem- 
bered that  the  osmotic  pressure  of  a  1  per  cent,  solution 
of  sodium  chloride  is  over  seven  atmospheres,  then  it  is 
obvious  that  the  strain  upon  the  tissues  of  an  organism 
suddenly  transferred  from  one  solution  to  another  of 
considerably  greater  or  less  salinity  may  easily  be  suffi- 
cient to  rupture  and  kill  them. 

Direct  observations  on  the  acclimatisation  of  the 
vertebrata  are  extremely  few,  except  in  the  case  of  cer- 
tain mammals  experimented  on  in  connection  with 
serum  therapeutics.  Davenport  and  Castle  *  have 
made  some  interesting  observations  on  the  acclimatisa- 
tion of  tadpoles  to  heat,  however.  Recently  laid  eggs 
of  Bufo  lentiginosus  were  divided  into  two  lots,  one  of 
which  was  allowed  to  develop  in  a  warm  oven  at  a  tem- 
perature of  24°  to  25°,  and  the  other  kept  at  15°. 

*  Loc.  cit. 


386  ADAPTIVE  VARIATIONS. 

After  four  weeks,  the  temperature  of  heat  rigor  was 
determined  by  gradually  heating  the  water  containing 
the  tadpoles.  Whilst  all  the  tadpoles  kept  at  15°  went 
into  heat  rigor  at  or  below  41°,  those  reared  at  25°  did 
not  in  any  case  die  at  a  temperature  below  43°,  the 
average  increase  of  resistance  amounting  to  3.2°.  This 
adaptation  to  higher  temperature  gradually  disappears 
on  returning  the  tadpoles  to  water  at  ordinary  tempera- 
atures,  more  than  half  of  the  3.2°  increase  being  lost 
after  keeping  them  for  17  days  at  15°. 

Probably  the  capacity  for  acclimatisation  is  present 
to  a  greater  or  less  degree  in  every  organism.  In  some 
observations  carried  out  at  Naples,*  I  found  that  the 
death  temperatures  of  a  Medusa  (Rhizostoma),  a  salp 
(Salpa  africana\  and  of  Amphioxus  were,  on  an  aver- 
age, respectively  1.3°,  .6°,  and  1.5°  higher  in  August 
than  they  had  been  in  April,  when  of  course  the  tem- 
perature of  the  sea  was  several  degrees  lower. 

The  adaptability  of  the  highest  organisms  to  changes 
of  environment  does  not  afford  so  much  support  to  our 
thesis — viz.,  that  adaptability  is  a  fundamental  prop- 
erty of  protoplasm — as  does  that  of  the  lowest  organ- 
isms, because  the  adaptation  is,  as  a  rule,  indirect  and 
complex.  Still  the  intrinsic  interest  of  the  subject  is 
so  great  as  to  warrant  a  brief  reference  to  it.  Almost 
all  of  the  exact  observations  deal  with  acclimatisation 
to  chemical  agents,  especially  the  toxins  secreted  by 
bacteria.  Upon  mice  Ehrlich  *  has  made  some  very 
exact  observations  on  adaptation  to  a  vegetable  poison, 
ricin.  The  mice  were  fed  on  food  cakes  soaked  in 

*  J.  Physiol.,  xxv.  p.  181,  1899. 

t  Deutsche  med.  Wockenschr.,  1891,  p.  976. 


ADAPTIVE  VARIATIONS.  387 

solutions  of  the  poison  of  increasing  strengths,  and 
after  feeding  for  various  lengths  of  time,  the  maximum 
amount  of  poison  the  animals  could  withstand  was  de- 
termined. This  amount  rapidly  increased  after  the 
first  day,  so  that  after  three  weeks'  feeding  it  was  found 
to  be  no  less  than  200  to  800  times  the  original  dose. 
Some  of  these  mice  were  then  kept  on  normal  food  for 
over  six  months,  and  at  the  end  of  that  time  could  still 
withstand  considerably  more  than  fifty  times  the  origi- 
nal amount  of  poison. 

Even  more  remarkable  results  have  been  obtained  in 
the  preparation  of  diphtheria  antitoxin.  For  this  pur- 
pose, Roux  *  uses  the  filtrate  from  diphtheria  bacillus 
cultures,  it  being  at  first  mixed  with  an  iodine  solution 
to  reduce  its  virulency.  One-quarter  cc.  of  the  iodised 
toxin  is  injected  on  the  first  day,  and  this  is  increased 
to  1  cc.  on  the  13th  day.  On  the  17th  day  \  cc.  of 
the  pure  toxin  is  injected,  and  this  is  gradually  in- 
creased in  amount  till  on  the  41st  day  10  cc.  is  in- 
jected, and  on  the  80th  day  no  less  than  250  cc. 
The  virulency  of  the  last  dose  must  have  been  some 
5000  to  10,000  times  greater  than  that  of  the  first  dose, 
and,  supposing  the  effect  produced  on  the  horse  was 
more  or  less  the  same  after  each  injection,  its  acclimati- 
sation to  the  toxin  must  have  increased  in  similar  pro- 
portion. As  is  well  known,  animals  can  be  acclimatised 
to  toxins  produced  by  other  bacteria,  such  as  those  of 
anthrax,  tetanus,  cholera,  typhoid,  plague,  and  likewise 
also  to  snake  venom;  but  it  is  unnecessary  to  refer  to 
these  here.  Upon  acclimatisation  in  man  there  are 
probably  no  exact  observations,  but  the  inexact  and  un- 
*  Vide  Crookshank's  "  Text-Book  of  Bacteriology,"  1896,  p.  58. 


388  ADAPTIVE  VARIATIONS. 

scientific  are  matters  of  common  personal  experience. 
A  hot  day  following  suddenly  on  a  long  spell  of  cold 
weather,  or  a  cold  one  on  a  long  spell  of  hot  weather, 
is  felt  much  more  keenly  than  days  of  considerably 
higher  or  lower  temperature  which  are  led  up  to  by  the 
gradual  change  of  the  seasons.  Likewise  also,  weather 
which  appears  very  hot  to  one's  self  will  be  looked 
upon  as  temperate  by  a  native  Indian,  or  even  an 
Anglo-Indian.  Acclimatisation  is  often  experienced 
by  those  who  indulge  in  excessive  amounts  of  alcohol, 
opium,  or  tobacco.  For  instance,  De  Quincey  was  at 
one  time  in  the  habit  of  taking  8000  drops  of  laudanum 
daily,  this  enormous  quantity  probably  producing  no 
greater  effect  than  a  dose  of  30  to  50  drops  in  an  ordi- 
nary man.  Again,  arsenic  eaters  are  able  to  swallow 
as  much  as  A  gm.  without  injury,  or  about  four  times 
the  ordinary  lethal  dose. 

These  various  observations  made  upon  members  of 
all  classes  of  the  Animal  and  Vegetable  Kingdoms  will, 
I  believe,  be  held  sufficient  proof  of  the  contention 
that  adaptability  is  present  in  all  organisms,  and  is 
therefore  a  fundamental  property  of  protoplasm. 
Whether  every  variation  produced  by  change  of  en- 
vironment is  in  the  direction  of  adaptation  to  the 
change,  it  is  of  course  impossible  to  say;  but  probably 
this  is  not  the  case,  as,  by  reason  of  the  close  correla- 
tion existing  between  many  of  the  characters  of  an 
organism,  the  change  may  produce  a  want  of  adapta- 
tion in  some  of  them,  but  an  increased  adaptation  in 
others.  Supposing,  however,  it  were  possible  to  esti- 
mate the  change  produced  in  every  character  in  the 
body,  it  seems  to  me  almost  certain  that  the  sum  total 


ADAPTIVE  VARIATIONS.  389 

of  all  the  changes  would  be  rather  in  the  direction  of 
adaptation  to  the  new  surroundings,  than  in  that  of 
non-adaptation.  It  is  not  to  be  supposed  for  a  moment 
that  every  one  of  a  group  of  organisms  exposed  to 
new  conditions  of  life  will  become  better  adapted  to 
them  than  any  one  of  the  group  had  been  originally; 
but  merely  that  the  characters  of  the  group  will,  on  an 
average,  become  better  adapted  than  they  had  been  be- 
fore. Doubtless  many  instances  can  be  thought  of  in 
which  the  effect  produced  by  a  change  of  environment 
has  no  appearance  of  being  in  the  least  adaptive,  but 
this  may  be  due  to  our  ignorance  of  what  constitutes  an 
adaptation.  For  instance,  it  may  be  asked  in  what 
way  a  starved  animal  is  better  adapted  to  semi-starva- 
tion than  a  well-nourished  one?  It  is,  of  course,  less 
adapted  in  that  it  has,  stored  up  in  its  body,  less  food 
material — such  as  fat  and  glycogen — on  which  it  can 
live,  but  it  is  obviously  better  adapted  in  that  its  metab- 
olism is  considerably  smaller  than  that  of  a  well- 
nourished  animal;  i.  e.,  it  actually  lives  on  considerably 
less  food.  Again,  it  may  be  asked  in  what  way  a  dusky 
coloured  Polyommatus  phlceas  is  better  adapted  to  a 
warm  climate  than  a  copper-coloured  one,  and  vice 
versa  with  reference  to  a  cold  climate  ?  Possibly  there 
is  nothing  adaptive  about  the  colour  of  the  wing  scales, 
but  doubtless  it  would  be  found  that,  on  an  average,  the 
dusky  butterflies  could  withstand  a  greater  degree  of 
heat  than  the  coppery  ones,  and  the  coppery  ones  a 
greater  degree  of  cold.  Hence  the  change  would,  on 
the  whole,  be  in  the  direction  of  adaptation. 

It  is  probable  that  somatic  variations,  by  reason  of 
their  adaptation  to  changed  surroundings,  are  of  very 


390  ADAPTIVE  VARIATIONS. 

great  importance  in  the  evolution  of  more  adaptive 
forms;  in  some  cases,  perhaps,  of  greater  importance 
than  genetic  variations.  Supposing,  for  instance,  a 
number  of  organisms  are  more  or  less  suddenly  exposed 
to  a  considerable  change  of  environment,  whereby  the 
majority  of  them  are  killed  off.  The  survivors  will  be 
those  which  had  the  greatest  power  of  adaptation  to 
the  new  surroundings,  and  though  the  somatic  varia- 
tions will  not  be,  as  such,  inherited,  yet  the  survivors 
will  be,  on  the  whole,  those  organisms  which  originally 
possessed  the  largest  proportion  of  the  particular  char- 
acters which  have  appeared  as  adaptive  somatic  varia- 
tions. That  is  to  say,  adaptive  somatic  variations  are, 
on  an  average,  a  magnified  image  of  similar,  but  much 
more  minute  genetic  variations,  and  hence  the  average 
hereditary  characters  of  the  survivors  are  in  the  direc- 
tion of  adaptation.  Again,  the  survivors  will  be  those 
individuals  possessing  the  largest  degree  of  innate 
adaptability  to  the  particular  environment  in  question. 
Hence  their  offspring  will  also  possess  this  adaptability, 
and  in  that  they  will  have  been  exposed  to  the  changed 
environment  throughout  the  whole  period  of  develop- 
ment, they  will  show  much  more  marked  somatic  varia- 
tions than  those  shown  by  their  parents.  Finally,  if  it 
be  admitted  that  the  effects  of  conditions  of  life  may 
be  in  some  degree  cumulative,  then  the  adaptation  of 
the  second  generation  to  the  environment  will  be  from 
this  cause  still  further  increased.  Views  somewhat 
similar  to  these  as  to  the  importance  of  somatic  varia- 
tions have  been  set  forth,  at  considerable  length  and 
with  admirable  lucidity,  by  Professor  Lloyd  Morgan  in 
his  work  on  "  Habit  and  Instinct  "  (p.  316),  and  to  this 


ADAPTIVE  VARIATIONS.  391 

the  reader  who  desires  more  detailed  discussion  is  re- 
ferred. 

Admitting  that  somatic  variations  are,  on  the  whole, 
adaptive,  and  admitting  also  to  a  very  limited  extent 
the  cumulative  influence  of  changed  conditions  of  life, 
are  we  to  agree  with  Henslow  *  that  the  close  adapta- 
tion of  plants  to  their  environment  is  due  entirely  to 
the  responsive  power  of  protoplasm  to  the  external  en- 
vironmental forces,  and  that  it  is  absolutely  unneces- 
sary to  call  in  the  aid  of  Natural  Selection?  By  no 
means.  Adaptive  variation  may  be  responsible  for  a 
good  deal  of  the  adaptation  observed  in  plants,  and  for 
a  very  small  part  of  that  observed  in  animals,  but  prob- 
ably in  each  case  by  far  the  larger  portion  must  be  as- 
cribed to  the  ever  present  and  ever  acting  agency  of 
Natural  Selection.  For  instance,  Henslow  argues  very 
plausibly  that  inasmuch  as  certain  plants  when  kept  in 
a  dry  atmosphere  develop  spines  and  other  characters 
similar  to  those  possessed  by  desert  plants,  it  is  valid  to 
conclude  that  these  desert  plants  owe  their  peculiar 
characters  to  the  direct  action  of  the  dry  hot  climate, 
and  to  that  alone.  Supposing  this  explanation  to  be 
correct,  however,  we  ought,  as  Wallace  points  out,f 
to  find  plants  with  spines  and  the  other  characteristics 
of  desert  plants  abounding  in  all  dry  countries,  but 
very  rare  or  wanting  in  moist  and  fertile  districts.  But 
this  is  by  no  means  the  case.  Wallace  states  that 
many  of  the  peculiarities  of  desert  plants  are  present 
in  the  flora  of  the  Brazilian  Campos,  and  in  that  of  the 
Galapagos  and  the  Sandwich  Islands,  but  very  few  of 

*Ibid.,  pp.  14  and  32. 

f  Nat.  Science,  vol.  v.  p.  177,  1894. 


392  ADAPTIVE  VAEIATIONS. 

the  plants  indeed  show  any  spines.  Again  spiny  plants 
are  exceedingly  rare  in  the  Canaries,  though  much  of 
the  surface,  owing  to  long  periods  of  drought,  presents 
the  conditions  which  elsewhere  are  supposed  to  produce 
spines.  Though  not  prepared  to  deny  that,  other 
conditions  equal,  aridity  may  favour  and  humidity 
check  the  growth  of  spines,  yet  Wallace  considers  that 
a  more  important  condition  lies  in  the  presence  or  ab- 
sence of  herbivorous  mammals,  against  whose  ravages 
the  spines  afford  protection.  Thus  he  mentions  sev- 
eral countries  which  are  not  particularly  arid,  but  in 
which  spiny  plants,  and  also  these  destructive  mam- 
mals, both  abound.  The  development  of  the  spines  is 
chiefly  dependent,  therefore,  on  the  action  of  Natural 
Selection,  and  is  not  a  direct  adaptation.  In  other 
cases  also  Wallace  believes  that  the  "  direct  action  of 
the  environment  can  have  produced  only  a  very  small 
portion  of  the  modifications  and  adaptations  that  actu- 
ally exist.  In  by  far  the  larger  number  of  cases  no 
such  explanation  is  possible,  and  no  other  adequate  ex- 
planation has  been  suggested  except  variation  and 
Natural  Selection." 

Though  it  seems  to  me  that  Wallace,  by  excluding 
all  other  agencies,  is  inclined  somewhat  to  exaggerate 
the  importance  of  Natural  Selection,  yet  his  explana- 
tion of  the  evolution  of  adaptive  forms  seems  much 
more  rational,  and  in  much  better  agreement  with 
facts,  than  that  given  by  Henslow.  The  view  to  which 
the  present  state  of  our  knowledge  seems  to  me  to  af- 
ford best  support  is  one  which  lies  more  or  less  between 
these  two  extreme  explanations.  It  is  most  con- 
veniently indicated  by  a  diagram.  Let  us  consider, 


ADAPTIVE  VARIATIONS. 


for  instance,  the  evolution  of  a  typical  aquatic  plant 
from  a  typical  terrestrial  one.  Supposing  it  were  pos- 
sible to  estimate  the  extent  to  which  characters  useful 
to  aquatic  life  were  present  in  a  group  of  terrestrial 
plants,  and  supposing  we  were  to  plot  out  the  fre- 
quency of  their  distribution,  then  this  might  take  the 
form  of  the  curve  given  in  the  extreme  left  of  the 
upper  portion  of  the  accompanying  diagram.  Here 


Distribution  of  characters  in  plants. 


alter  exposure  for        after  i 
typical  one. generation  to  for  many 

terrestial  plant:      *quoous  environment:         generations. 


aquatic  pU 


plant. 


Distributior 


of  eha  -acters 


animals. 


10 


20 


80 


90 


1.08 


80  40  50          60  7.0 

Percentage  of  aquatic  characters. 

FIG.  30. — Evolution  of  the  aquatic  plant  and  the  aquatic  animal. 

we  see  that  the  most  frequently  occurring  plant  had 
10  per  cent,  of  "  aquatic "  characters,  the  extremes 
ranging  from  0  to  20  per  cent.  Supposing  now  this 
group  of  plants  were  exposed  for  one  generation  to  an 
aqueous  environment.  It  would  be  found  at  the  end  of 
that  time  that  the  proportion  of  aquatic  characters  had 
considerably  increased,  say  to  26  per  cent.,  but  the  fre- 
quency of  distribution  of  the  characters  about  the 
mean  would  still  be  symmetrical  as  it  was  before,  the 
extremes  now  varying  from  14  to  38  per  cent.  Some  of 
the  plants,  therefore,  would  still  possess  fewer  aquatic 


394  ADAPTIVE  VARIATIONS. 

characters  than  were  possessed  by  a  small  number  of 
the  original  group  of  plants,  in  accordance  with  Dar- 
win's dictum  concerning  plants  that  "  whether  the 
station  (they  inhabited)  was  unusually  dry  or  humid, 
variations  adapting  them  in  a  slight  degree  for  directly 
opposite  habits  would  occasionally  arise."  It  will  be 
noticed  that  in  the  diagram  the  curve  of  distribution  of 
the  characters  is  made  slightly  more  flat  topped  than  the 
other  curves,  indicating  that  the  variability  of  a  group 
of  plants  suddenly  exposed  to  a  changed  environment 
is  increased.  Supposing  that  this  group  of  plants  is 
exposed  to  the  aqueous  environment  for  a  number  of 
generations,  then,  through  the  cumulative  action  of  con- 
ditions of  life,  the  adaptation  will  become  considerably 
increased,  and  the  plants  will  now  show,  on  an  average, 
say  40  per  cent,  of  the  aquatic  characters  of  a  typical 
aquatic  plant.  This  increase  of  adaptation  from  the 
stage  reached  after  one  generation  is  supposed  to  be 
more  or  less  permanent  and  hereditary,  or  would  still 
be  present  if  the  plants  were  returned  to  their  original 
dry  land  environment.  But,  however  many  genera- 
tions the  plants  be  kept  in  their  watery  surroundings, 
it  is  supposed  that  they  will  never  become  adapted  to 
it  like  typical  aquatic  plants.  In  order  to  evolve  such 
plants,  Natural  Selection  must  be  present  in  addition, 
and  in  this  case  the  distribution  of  the  plants,  in  respect 
of  aquatic  characters,  will  ultimately  arrive  at  that  in- 
dicated in  the  curve  on  the  extreme  right  of  the 
diagram. 

The  lower  half  of  the  diagram  is  meant  to  represent 
the  evolution  of  an  aquatic  animal,  such  as  a  mammal, 
from  a  land  animal.  Such  an  animal  would  in  the  first 


ADAPTIVE  VARIATIONS.  395 

place  have  very  few  characters  adapting  it  to  an  aquatic 
existence,  and  so  the  curve  of  distribution  of  such  char- 
acters will  be  a  more  steeply  sloped  one  than  that  for 
plants.  Also  the  direct  effect  of  environment  in  the 
direction  of  adaptation  will  be  very  much  less  than  in 
the  case  of  plants,  even  after  exposure  for  a  large  num- 
ber of  generations.  In  fact,  to  effect  any  real  and  con- 
siderable change  it  will  be  essential  to  call  in  the  aid 
of  Natural  Selection,  and  this,  by  acting  constantly  for 
a  very  large  number  of  generations^  will  gradually 
evolve  a  typical  aquatic  mammal  such  as  the  seal,  dol- 
phin, or  whale. 

In  spite  of  all  that  has  been  written  to  account  for 
the  almost  universally  present  adaptation  which  we 
see  in  animate  nature,  there  is  still  a  lingering  doubt 
in  the  minds  of  many  men  as  to  the  entire  adequacy  of 
the  explanations  hitherto  offered.  It  is  a  feeling  such 
as  this  which  prompted  Weismann  to  formulate  an  ad- 
ditional principle  in  explanation  of  adaptation,  and  of 
other  phenomena,  as  the  degeneration  of  disused  organs, 
viz.,  his  theory  of  Germinal  Selection.*  This  theory 
supposes  that,  similar  to  the  struggle  for  existence  ex- 
perienced by  individual  organisms,  so  there  is  a  strug- 
gle among  the  determinants  of  the  germ-plasm  of  each 
single  individual  to  obtain  as  great  a  supply  of  nutri- 
ment as  possible,  and  so  flourish  at  the  expense  of 
weaker  determinants.  Supposing,  for  instance,  that 
parts  of  the  body,  such  as  the  hinder  extremities  of  the 
quadruped  ancestors  of  our  common  whales,  are  ren- 
dered useless.  As  selection  ceases,  individuals  with 

*"Ueber  Germinal  Selection,"  Jena,  1896  (English  translation, 
Chicago,  1896). 


396  ADAPTIVE  VAEIATIONS. 

small  hind  legs,  represented  (Weismann  supposes)  by 
weaker  determinants  in  the  germ,  are  as  favourably 
placed  in  the  struggle  for  existence  as  those  with  large 
hind  legs,  represented  by  stronger  germ  determinants. 
The  weaker  determinants,  in  their  struggle  with  the 
other  determinants  which  represent  useful  organs  in  the 
body,  will  be  worsted,  and  gradually  become  more  and 
more  enfeebled,  the  hind  legs  which  they  represent  be- 
coming correspondingly  smaller  and  smaller  till  they 
finally  disappear  altogether.  Supposing,  on  the  other 
hand,  that  the  individuals  showing  a  greater  develop- 
ment of  any  particular  characters  than  the  average  are 
for  this  reason  favoured  by  Selection,  then  the  determi- 
nants representing  these  characters  in  the  germ-plasm 
will  also  be  more  powerful  than  the  average,  and  by  ab- 
sorbing more  nutriment  will  become  still  more  robust, 
and  produce  descendants  exhibiting  the  characters  in 
an  increased  degree.  That  is  to  say,  the  descendants 
will,  by  Germinal  Selection,  become  more  and  more 
adapted  to  the  conditions  in  respect  of  which  they  were 
originally  favoured  by  Natural  Selection. 

This  theory,  though  plausible  enough,  is  absolutely 
opposed  to  fact  in  so  far  as  it  relates  to  the  evolution  of 
more  adaptive  forms.  As  we  have  seen  in  Chapter 
IV.,  so  far  from  the  individuals  selected  in  respect  of 
any  character  tending  to  transmit  that  character  in  in- 
creased strength  to  their  descendants,  they  almost  in- 
variably transmit  less  of  it,  or  the  offspring  show,  on  an 
average,  a  greater  or  less  degree  of  regression  towards 
mediocrity,  according  to  the  amount  of  the  character 
present  in  their  more  remote  ancestors. 

The  degeneration  of  disused  organs  is,  it  must  be  ad- 


ADAPTIVE  VARIATIONS.  397 

mitted,  a  difficulty  which  has  never  been  hitherto  ade- 
quately accounted  for,  and  hence,  in  lieu  of  something 
better,  Weismann's  hypothesis  may  in  this  respect  be 
provisionally  accepted.  Still  it  is  always  to  be  remem- 
bered that  it  is  no  more  than  an  hypothesis,  which  has 
not,  and  never  can  have,  any  experimental  evidence 
to  support  it. 


AUTHOR'S  INDEX. 


A 


Ainsworth,  330 

Allen,  J.  A.,  4,  7,  325,  326,  327, 
328 

B 

Bachman,  353 

Baldwin,  M.,  223 

Bateson,  W.,  37,  41,  51,  52,  53, 
54,  55,  57,  59,  64,  91,  92,  137, 
160,  272,  273,  275,  277,  278 

Baxter,  93 

Beddard,  317 

Beddoe,  85,  93 

Beeton,  347,  348 

Bennett,  69,  163 

Bertillon,  118 

Beudant,  383 

Blankinship,  41,  42,  44 

Bonnier,  312,  313,  353,  374 

Boscher,  E.,  260 

Bosnian,  355 

Bowditch,  202,  206 

Bramley-Moore,  L.,  87 

Brandes,  G.,  295 

Bravais,  74 

Breman,  G.  A.,  314 

Brewer,  354 

Brewster,  24,  28 

Britton,  60 

Broca,  71 

Brooks,  W.  K.,  114 

Brown,  60 

Browne,  E.  T.,  30 

Brown-Sequard,  362,  363,  364 

Brucke,  255,  256 

Bullard,  25 

Bumpus,  27,  206,  212,  214,  215, 
216,  341,  343,  345,  370 

Burnes,  330 


Castle,  381,  385 

Claus,  274 

Clayton,  247 

Cockerell,  T.  A.  D.,315 

Coldstream,  254 

Cooke,  232,  287 

Cope,  325,  354,  357,  362 

Correns,  155,  157,  159,  160 

Costa,  314 

Costantin,   265,    266,    267,  374, 

376 

Cox,  R,  290 
Crookshank,  387 
Cunningham,   J.   T.,   251,  340, 

341 

Cuvier,  294 
Czerny,  382 


Dallinger,  379,  380 

Dareste,  173,  174 

Darwin,  C.,  2,  53,  58,  59,  65,  66, 
68,  69,  70,  85,  96,  97,  114,  139, 
140,  141,  148,  161,  166,  172, 
185,  186,  187,  197,  219,  221, 
222,  281,  293,  311,  317,  329, 
330,  331,  332,  333,  335,  336, 
352,  353,  354,  355,  360,  371, 
372,  394 

Davenport,  C.  B.,  25,  36,  41,  42, 
44,  90,  213,  245,  246,  247,  381, 
384,  385 

Delage,  Y.,  295,  360 

Detmer,  286 

Dixey,  167,  230,  238,  239 

Dorfmeister,  233,  240 

Driesch,  H.,  336 

Duchartre,  264 

Duncker,  G.,  14,  26,  27,  28,  34, 
36,  83,  84,  215 


400 


AUTHOR'S  INDEX. 


E 


Edmondston,  295 

Ehrlich,  386 

Eigenmann,  253,  324 

Eimer,  233,  234,  239,  240,  250, 

268,  269,  287,  288,  314,  362 
Elliott,  S.,  248 
Engleheart,  163 
Erman,  354 
Errera,  377 
Ewart,  J.  C.,  113,  141,  148,  152, 

168,  169,  176 


Falconer,  330,  355 
Faxon,  254 
Filon,  84 
Finn,  F.,  176 
Fischel,  207 
Fischer,  232,  238,  239 
Flahault,  312 
Fletcher,  W.  H.  B.,  92 
Focke,  161,  163,  166 


Haacke,  143 

Heape,  W.,  119,  120,  121 

Hefferan,  35 

Hegler,  R.,  375 

Heincke,  F.,  322,  323,  324 

Hellriegel,  262 

Hennig,  201 

Hensen,  V.,  115,  118 

Henslow,  G.,  264,  265,  266, 

372,  376,  391,  392 
Herbert,  161,  163 
Herbst,  C.,  55 
Heron,  R.,  64,  148 
Hertwig,  O.,  227 
Hewitt,  353 
Higginbottom,  225 
Hill,  L.,  363,  364 
His,  201 
Holmgren,  295 
Humphreys,  58 
Hunger,  377 
Hunter,  J.,  295 
Hurst,  165,  166 


G 

Gain,  263 

Galton,  F.,  7,  13,  17,  19,  20  22 
52,  65,  74,  75,  76,  84,  116,  118,' 
122,  123,  124,  125,  126,  127 
128,  129,  130,  132,  133,  134, 
149,  151,  152,  170,  171,  172, 
208,  356 

Garstang,  25,  319,  321,  322 

Gartner,  69,  159 

Gauss,  10 

Geddes,  P.,104,  287 

Giard,  40,  41 

Gibbons,  278 

Gilbert,  H.,  282,  284 

Godron,  267,  374 

Goebel,  K. ,  246 

Goss,  H.,  290 

Gregson,  289 

v.  Guiata,  142 

Gulick,  315 

GUnther,  R.  T.,  274 


Jameson,  H.  L.,  350,  351 
Jordan,  70 
Jourdain,  S.,  254 


Karsten,  247,  374 
Kerner,  164,  165 
Knight,  A.,  281 
Knowlton,  225,  227 
Koch,  G.,  268,  288 
Kohlbriigge,  221 
Kolreuter,  159,  161,  162 
Koppen,  228 
Kraatz,  54 
Kriechbaumer,  54 
Krocker,  294 
Kropotkin,  P.,  253 


Lawes,  J.,  282,  284 
Lecoq,  161 
Lee,  82,  87 


AUTHOR'S  INDEX. 


401 


Lesage,  269,  270,  353,  374 

Leydig,  268,  314 

Lillie,  225 

Lindley,  376 

Linton,  164 

List,  252 

Lister,  Lord,  255,  256 

Loeb,  J.,  278 

Lothelier,  264,  374 

Ludwig,  15,  46,  48 

M 

MacCulloch,  254 

MacLeod,  285 

MacMunn,  251 

Massart,  382 

Mayer,  A.  G.,  90 

Meehan,  311 

Meldola,  260,  291 

Mendel,  G.,  155,  156,  157,  158, 

159,  160,  169 
Menetries,  295 
Merrifield,  234,  235,  239,  240, 

241,  242,  243,  268 
Metzger,  352 
Milardet,  162 
Millais,  E.,  123,  176 
Milne-Edwards,  54 
Minot,  24, 201,  203,  204,  205,  208 
Mobius,  231 
Montgomery,  219,  220 
Moreau,  J.,  116 
Morgan,  L.,  223,  373,  390 
Moulton,  F.,  67 

N 

Nathusius,  294 
Neal,  381 
Newman,  289,  290 
Nicoll,  257 
Noll,  245 

O 

Obersteiner,  364 


Packard,  254,  274 
Pallas,  354 


Pearson,  K,  26,  30,  32,  33,  39, 
80.  82,  84,  87,  88,  89,  90,  124, 
125,  127,  128,  133,  135,  136, 
137,  150,  151,  153,  176,  180, 
181,  182,  185,  205,  210,  211, 
346,  347,  348 

Petersen,  27 

Pfitzner,  221 

Planta,  A.  von,  287 

Pledge,  J.  H.,  16,  30 

Pouchet,  256 

Poulton,  E.  B.,  243,  244,  251, 
255,  257,  258,  259,  260,  261, 
291 

Preyer,  201 

Q 

Quetelet,  12,  13 
Quincey,  De,  388 

R 

Ray,  375,  378 

Richter,  382 

Ridgway,  219 

Rimpau,  166 

Rolfe,  163,  164 

Romanes,  G.  J.,  66,  67,  70,  120, 

121,  363 
Ross,  J.,  243 
Roux,  387 


3 


Sachs,  245,  246 
Salisbury,  Lord,  336 
Sargeant,  271 
Sauermann,  293 
Saunders,  E.  A.,  64,  160 
Schimper,  245 
Schmankewitsch,  271,  272,  273, 

274,  275 

Schubeler,  312,  313 
Schwalbe,  221,  222 
Scott,  70 

Sedgwick,  A.,  184,  210,  211 
Semper,  K.,  255,  256,  302,  303, 

304,  305 
Smith,  M.,  70 
Sorauer,  249 


402 


AUTHOR'S  INDEX. 


Stahl,  248 

Standfuss,    167,   171,   172,   175, 

230,  236,  238,  240,  268 
Steinert,  92 
Strasburger,  245 
Struthers,  58,  172 


Tawell,  J.  A.,  290 
Tegetmier,  85 
Thompson,  H.,  14,  82,337 
Thomson,  A.,  104,287 
Tschermak,  155,  159 


Varigny,  H.  de,  267,  294,  302, 
303,  304,  305,  306,  307,  384 

Verschaeffelt,  28 

Vines,  S.  H.,  228 

Vire,  252 

Vochting,  H.,15 

Vries,  H.  de,  15,  29,  33,  50,  59, 
60,  61,  62,  63,  114,  155,  159, 
162,  171, 199,  200,  228,  284 

W 

Wagner,  M.,  317 

Wallace,  A.  R,  7,  66,  67,  259, 

293,  316,  317,  335,  336,  391, 

392 


Wallace,  149 

Walsh,  96 

Warren,  E.,  8, 14,  26,  34,  81,  83, 
84,  97,  178,  179,  180,  308,  309 

Weisbach,  24 

Weismann,  A.,  101,  102,  103, 
114,  115,  116,  118,  134,  142, 
169,  177,  178,  180,  197,  223, 
233,  235,  236,  238,  239,  240, 
241,  335,  355,  356,  358,  360, 
364,  366,  395,  396,  397 

Welch,  F.  H.,  244 

Weldon,  W.  F.  R.,  13,  21,  23, 
26,  39,  40,  77,  79,  80,  81,  82, 
97,  160,  206,  285,  286,  318,  337, 
338,  339,  340,  341,  345,  346 

Westphal,  364,  365 

Whitfleld,  307 

Wichura,  164 

Wiesner,  246 

Wilson,  J.  H.,  163 

Windle,  B.,  174 

Wittich,  255,  256 

Wood,  T.  W.,  258 


Yule,  G.  U.,  84,  348 

Yung,  249,  271,  292,  306,  307 


SUBJECT  INDEX. 


Aberrations,  among  Lepidoptera, 
237 

Abnormalities,  spontaneous  ori- 
gin of,  57,  59 

Abyssal,  light,  theory  of,  254; 
fauna,  254 

Acacia,  interchange  of  root  and 
stem  in,  376 

Acceleration  of  growth,  persist- 
ence of,  204,  207 

Acclimatisation,  to  high  tem- 
perature, 379;  to  saline  solu- 
tions, 382;  in  tadpoles,  385;  in 
mice,  386;  in  horse,  387;  in 
man,  387 

Acerina  cernua,  measurements 
of,  14 

Acquired  Characters,  in  lowland 
and  highland  plants,  811;  in 
spiderwort,  313 

Acquired  Characters,  heritable- 
ness  of,  352;  in  maize,  352;  in 
cress,  353;  in  ducks,  353;  in 
sheep,  354;  in  dogs,  355;  in 
butterfly,  356;  in  guinea-pigs, 
363;  in  mould,  377,  378 

Adaptability,  a  property  of  pro- 
toplasm, 373,  387;  innate,  390 

Adaptation,  want  of,  as  a  cause 
of  variability,  217,  218;  sea- 
sonal, in  Lepidoptera,  234,  241 ; 
in  mammals,  243;  in  aquatic 
and  terrestrial  plants,  266;  in 
maritime  plants,  270;  of  di- 
gestive organs  to  food,  294, 295; 
of  plants  to  changed  climate, 
311,  312;  and  definite  varia- 
tions, 373 


Adaptive  variations,  371;  in 
plants,  374;  in  moulds,  375, 
377,  378;  in  sunflower,  375;  in 
acacia,  376;  in  willow,  376;  in 
brambles,  376;  in  Flagellata, 
379;  in  Stentor,  381;  in  other 
Protozoa,  382;  in  Mollusca, 
383;  in  Actinia,  384;  in  tad- 
poles, 385;  in  mice,  386;  in 
horse,  387;  in  man,  387;  con- 
cern average  variations,  389; 
do  not  render  natural  selection 
unnecessary,  391;  explained 
by  germinal  selection,  395 

Aerial  and  aquatic  plants,  265 

Aggressive  resemblance,  255 

Ainos,  correlation  in  skeletons 
of,  82 

Alpine  plants,  compared  with 
lowland,  311 

Alpine  climate,  effect  of,  on 
plants,  311 

Alternative  heritage,  132,  156 

Ammonia,  effect  of,  on  larvae, 
301 

AmcEbse,  acclimatisation  of,  to 
saline  solutions,  382 

Amphibious  plants,  265 

Amphidasys  betularia,  variation 
in,  91 

Amphimixis,  103,  114 

Analogous  Variation,  96 

Ancestors,  impress  individuality 
on  sex-cells,  134 

Ancestral  Heredity,  Law  of,  122 

Ancon  sheep,  origin  of,  58,  172 

Animals,  hybrids  among,  167, 
168 

Anthropometric  data,  analyses 
of,  122 


403 


404 


SUBJECT  INDEX. 


Anthropometric  measurements, 
obey  Laws  of  Chance,  12,  13; 
variability  in,  24 

Aphides,  measurements  upon, 
179 

Aquatic  plants,  265;  evolution 
of,  393 

Aral  Sea,  cockle  present  in,  275 

Arctic  climate,  effect  of,  on  mam- 
mals, 243;  effect  of,  on  plants, 
313 

Arithmetical  mean  error,  deter- 
mination of,  23 

Artemia,  the  effect  of  salinity  on, 
271,  274 

Asexual  reproduction,  in  an 
ostracod,  177;  in  daphnia,  178; 
in  aphis,  179;  in  plants,  183; 
in  potato,  184;  in  apple,  185 

Assertive  mating,  increases  cor- 
relation, 151 

Asymmetrical  curves  of  distribu- 
tion, 29;  obtained  by  expand- 
ing binomials,  31 

Asymmetrical  series,  fitted  with 
calculated  curves,  30;  types 
of,  33 

Asymmetry,  index  of,  33 

Atavism,  139 

Aurelia  aurita,  variations  in,  30 


B 


Babies,  variability  of,  205 
Basset  hounds,  records  of,  122 
Bees,  effect  of  nutrition  on,  287 
Bertillon's   system,    useless   for 

identical  twins,  118 
Binomial,  expansion  of,  31 
Binomial  curve,  approximation 

of,  to  probability  curve,   12; 

types  of,  32 
Biophors,  102 
Birds,  effects  of  foods  on,  293; 

geographical  races  of,  325 
JttscuteUa   Icevigata,  discontinu- 
ous variation  in,  64 
Bisexual  descent,  controlled  by 

Law  of  Heredity,  134 
Blastogenic  variations,  101,  242, 

244 


Blended  heritage,  132 

Brain,  diminished  by  disuse,  332 

Branchipus,  in  relation  to  Ar- 
temia,  274 

Brothers,  relation  between,  128, 
135;  hereditary  resemblance  be- 
tween, 151;  correlation  be- 
tween, 347 

Bud- Variation,  in  nectarine,  185; 
in  Chrysanthemum,  186;  in 
moss-rose,  186;  causes  of,  186; 
probably,  discontinuous  varia- 
tion, 187 

Buttercup,  variation  in  petals  of, 
29 


Canidse,  variation  in,  327 

Carcinus  mosnas,  asymmetry  in 
measurements  of,  39;  correla- 
tion in,  80;  natural  selection 
in,  337 

Cardium  edule,  effect  of  salinity 
on,  275 

Castration,  effect  of,  on  animals, 
95 

Caterpillars,  protective  resem- 
blance in,  260 

Cave  animals,  251,  253,  254 

Centroid  vertical,  defined,  17 

Cervical  sympathetic  nerve,  in- 
herited effects  of  section  of, 
363 

Characters,  new,  produced  by 
crossing,  161 ;  acquired,  herit- 
ableness  of,  352  et  seq.;  in 
guinea-pigs,  363 

Chlorophyll,  colouring  matter  of 
larvae,  291 

Chrysalides,  resemblance  of,  to 
surroundings,  258 

Chrysanthemum  leucanthemum, 
variation  in,  46 

Chrysanthemum  tsegetum,  varia- 
tion in,  15;  origin  of  many- 
rayed  form  of,  63 

Civilisation,  effect  of,  on  correla- 
tion, 82 

Climate,  producing  local  races, 
311,  327;  effect  of,  on  hairy 
covering  of  animals,  329 


SUBJECT  INDEX. 


405 


Clover,  variation  in  blossoms  of, 
29;  five-leaved,  origin  of,  62; 
five-leaved,  effect  of  soil  on, 
284 

Coat  colour,  inheritance  of,  133 

Cockle,  effect  of  salinity  on, 
275 

Coefficient  of  Regression,  127, 
128,  132;  relation  of,  to  corre- 
lation, 128 

Coefficient  of  variation,  26;  is 
this  a  correct  measure  of  varia- 
bility? 27 

Cold,  effect  of,  on  Lepidoptera, 
233 

Collateral  inheritance,  125,  135; 
relation  of,  to  lineal,  128 

Colour  of  skin,  changes  in,  pro- 
duced by  light,  251,  256,  257 

Colouration,  of  flounder,  effect 
of  light  on,  251;  of  pupae,  259; 
of  caterpillars,  260 

Columba  lima,  reversion  to  char- 
acters of,  140,  141 

Compositse,  variation  in,  47 

Conception,  condition  of  mother 
at  time  of,  as  regards  offspring, 
209 

Conditions  of  life,  may  produce 
local  races,  311;  effect  of,  on 
Lepidoptera,  316;  cumulative 
action  of,  in  maize,  352;  in 
cress,  353;  in  ducks,  353;  in 
sheep,  354;  in  dogs,  355;  in 
butterfly,  356 

Correlated  variation,  in  Sciurus 
carolinensis,  6;  in  Linaria 
spuria,  16 

Correlation,  determination  of,  74; 
improved  method,  84;  capri- 
ciousness  of,  86;  between  ances- 
tors, 125;  relation  of,  to  regres- 
sion, 128;  in  man,  75,  83,  150, 
347,  348;  in  shrimps,  79;  in 
crabs,  80,  81;  in  prawn,  82;  in 
Vegetable  Kingdom,  135;  in 
daphnia,  178;  in  aphis,  180;  in 
Ficaria,  185;  relation  of,  to  va- 
riability, 210;  homotypic,  211 

Correlation  constant,  74 

Correlation,  negative,  73,  79 


Correlation  values,  diagram  of, 
78 

Corrosive  sublimate,  acclimatisa- 
tion to,  381 

Cowslip,  hybrids  of,  164 

Crabs,  measurements  of,  13,  14; 
natural  selection  in,  337 

Critical  period  of  reaction,  in 
Lepidoptera,  240 

Crops,  growth  of,  affected  by 
manures,  282 

Crosses,  between  Echinoderms, 
110;  fertility  of,  165;  among 
Lepidoptera,  172 

Crossing  of  type  and  varietal 
forms,  92 

Crowfoot,  effect  of  nutrition  on, 
284 

Crustacea,  effect  of  light  on,  253, 
254;  effect  of  salinity  on;  271, 
275 

Cultivation,  effect  of,  on  spider- 
wort,  313 

Cumulative  action  of  conditions 
of  life,  in  maize,  352;  in  cress, 
353;  in  ducks,  353;  in  sheep, 
354;  in  dogs,  355;  in  butterfly, 
356;  in  general,  394 

Cumulative  action  of  products 
of  metabolism,  308 

Curve  of  Error,  normal,  18; 
values  of,  compared  with  act- 
ual measurements,  22 

Curves  of  plant  variation,  43,  44, 
45,  49,  50 

Curves,  symmetrical  and  asym- 
metrical, represented  by  gen- 
eralized expression,  32 

Cypris  reptans,  parthenogenesis 
in,  177 


Daffodil,  crosses  of,  163 

Daphnia  magna,  measurements 
upon,  178;  reaction  of,  to  en- 
vironment, 179;  effect  of  prod- 
ucts of  metabolism  on,  308 

Darkness,  effect  of,  on  plant 
growth,  245;  on  molluscs,  252; 
on  Crustacea,  253;  on  fish,  254 


406 


SUBJECT  INDEX. 


Daylight,  effect  of,  on  plants, 
246;  on  flounder,  251 

Death,  ages  at,  in  man,  347 

Death  rate,  selective,  in  crabs, 
338;  in  sparrows,  341 

Definite  variations,  371 

Degeneration,  explained  by 
germinal  selection,  396 

Desert  plants,  263 

Destruction,  selective,  in  crabs, 
337;  in  sparrows,  341;  in  mice, 
351 

Determinants,  102;  affected  by 
temperature,  236;  necessity  of 
existence  of,  358;  and  germinal 
selection,  395 

Determinate  variations,  373 

Development,  of  Echinoderm 
larvae,  effect  of  temperature  on, 
190,  194;  rate  of,  in  man,  201; 
diminution  of  variability  with, 
204,  206;  stages  of,  in  relation 
to  reaction  to  environment, 
190 

Digits,  variation  in,  56 

Dimorphism,  in  earwig,  38;  in 
crab,  39;  in  fish,  42,  in  marsh 
plant,  42;  origin  of,  64,  65.; 
seasonal,  in  Lepidoptera,  233, 
239,  241,  242 

Diplogenesis,  theory  of,  357 

Discontinuous  variations,  origin 
of,  51 ;  importance  of,  in  evolu- 
tion, 52;  as  regard  bud-varia- 
tions, 187 

Disease,  affects  blonds  more 
than  brunets,  93 

Diseases,  affect  identical  twins 
similarly,  116,  117,  118 

Disuse,  effect  of,  on  skull,  332; 
on  pigeon  bones,  333;  on  duck 
bones,  333;  inheritance  of  ef- 
fects of,  360 

Dogs,  effect  of  Indian  climate 
on,  355 

Domestication,  effect  of,  on  vari- 
ability, 221 ;  on  rabbit,  331 

Dominant  characters  in  hy- 
brids, 156,  159 

Dryness  of  soil,  effect  of,  on 
plants,  263 


Duck,   effect   of   domestication 
on,  333 


E 


Earwig,  variation  in  forceps  of, 
37 

Echinoids,  crosses  among,  168; 
specific  metabolism  of,  296, 
298 

Echinoid  larvae,  effect  of  tem- 
perature on  size  of,  229 

Elimination,  of  sparrows  in 
storm,  341;  of  gray  mice,  350 

Embryos,  rate  of  growth  of  hu- 
man, 201;  variability  of,  206 

Environment,  diminishing  ef- 
fect of,  with  development,  195; 
variable  reaction  of  organisms 
to,  197;  relation  of,  to  develop- 
ing plants,  199;  effect  of,  de- 
pendent on  rate  of  growth,  203; 
permanent  effect  of,  207;  un- 
favourable, as  a  cause  of  vari- 
ability, 218;  saline,  as  a  direct 
cause  of  variations,  277;  may 
produce  local  races,  310;  action 
of,  through  internal  secretions, 
358;  may  affect  determinants 
of  germ-plasm,  366;  does  little 
without  natural  selection,  392 

Environment,  effect  of,  on 
growth,  184,  194;  on  variabil- 
ity of  sparrow,  214;  on  varia- 
bility of  periwinkle,  216;  on 
variability,  218,  220;  on  repro- 
ductive system,  221 ;  on  soma, 
223;  on  colour  of  chrysalides, 
258;  on  Lepidoptera,  316;  on 
plants,  311;  on  oyster,  314; 
on  snail,  314;  on  Porto  Santo 
rabbit,  331 

Epilepsy,  inheritance  of  acquired, 
364 

Equable  Variability,  Law  of,  96 

Equipotency  of  parental  herit- 
ages, 150 

Error  of  Mean  Square,  26 

Evolution,  importance  of  varia- 
tion in,  336;  of  aquatic  plant, 
393 


SUBJECT  INDEX. 


407 


Exclusive  inheritance,  132;  obeys 
Law  of  Heredity,  133 

Excreta,  effect  of,  on  growth,  of 
larvae,  298;  of  molluscs,  302, 
307;  of  tadpoles,  306;  of  Daph- 
nia,  308 

Exophthalmos,  apparent  inherit- 
ance of,  263 

Expectation  of  life,  347 

Exposure,  effect  of,  on  plant 
growth,  248 

Eye-colour,  transmission  of,  132 

Eyes,  relation  of,  to  change  of 
skin  colour,  255 

F 

Fertilisation,  premature,  in  rab- 
bits, 113;  effect  of  tempera- 
ture at  time  of,  191 

Fertilisations,  artificial,  method 
of  effecting,  105 

Fertility,  correlation  of,  with 
structure,  87,  88,  89;  inherit- 
ance of,  in  man  and  in  horse, 
87;  maximum,  of  type  forms, 
90;  of  hybrids,  165;  in  man,  348 

Fibonacci,  series  of,  48 

Finger-print  method,  as  regards 
twins,  118 

Finger-prints,  170 

Fin-rays,  variability  of,  in  cer- 
tain fishes,  27 

Fit  of  curves,  estimation  of,  35 

Flagellata,  adaptation  of,  to  high 
temperature,  379 

Floral  structures,  relation  of,  to 
arid  surroundings,  265 

Flounder,  effect  of  light  on,  251 

Food,  as  a  cause  of  variability, 
281;  effect  of,  on  bees,  287;  on 
aphides,  288;  on  Lepidoptera, 
288-290;  on  larvae,  291;  on  tad- 
poles, 292;  on  birds,  293,  295; 
on  mammals,  294;  ouSaturnia, 
317 

Forficula  auricularia,  variation 
in,  37 

Fowls,  production  of  malforma- 
tions in,  174;  effect  of  meat 
on,  295;  effect  of  domestication 
on,  333 


Fox,  variation  in,  328 

Fraternal  correlation,  128,  135; 
in  aphis  and  daphnia,  180 

Frog,  development  of,  in  rela- 
tion to  temperature,  225,  226, 
227 

Frogs,  effects  of  feeding  on,  292 


G 


Gallus  bankiva,  reversion  to 
characters  of,  142 

Galton's  function,  74 

Gangrene,  apparent  inheritance 
of,  363 

Genetic  Selection,  81 

Genetic  variations,  102 

Germ,  infection  of,  175 

Germinal  Selection,  395 

Germinal  variations,  102 

Germ-plasm,  existence  of  definite 
units  in,  135;  influenced  by 
somatic  variations,  360 

Geographical  races,  variability 
of  species  with,  220;  of  mack- 
erel, 319;  of  herring,  322;  of 
birds,  325;  of  mammals,  327 

Grades,  method  of,  20,  22 

Grandparents,  contribution  of, 
to  characters  of  offspring,  122 

Growth,  rate  of,  in  man,  201;  in 
guinea-pig,  203;  relative  rate 
of,  203;  retardation  and  accel- 
eration of,  204;  loss  of  varia- 
bility with,  204;  curves  of, 
208;  in  offspring,  affected  by 
mother,  209;  effect  of  salinity 
on,  278;  in  relation  to  prod- 
ucts of  metabolism,  296,  298, 
302,  303,  307,  308 

Growth  of  organisms,  reaction 
of,  to  environment,  189 

Growth  power,  progressive  loss 
of,  203 

Gull,  effect  of  food  on,  295 


Hsematoma,  apparent  inheritance 

of,  363 
Hair,  effect  of  cold  on,  244 


408 


SUBJECT  INDEX. 


Hare,  effect  of  cold  on,  244;  vari- 
tion  in,  329 

Heredity,  decided  at  time  of 
fertilisation,  116,  119;  Law  of 
Ancestral,  122;  solution  of 
problem  of,  125;  a  phase  of 
homotyposis,  137;  intensity  of, 
in  relation  to  variability,  211 

Heritage,  alternative,  156 

Heritages,  unequal  from  parents, 
150;  absolute  amounts  of,  in 
offspring,  153 

Herring,  local  races  of,  322 

Homoeosis,  in  Saw-fly,  Humble 
bee,  Crustacea,  54 

Homologous  organs,  correlation 
of,  72 

Homotypes,  136 

Homotyposis,  136;  relation  of, 
to  variability,  211 

Horses,  trotting,  prepotency  in, 
149 

Humidity,  effect  of,  on  plant 
growth,  262;  producing  local 
races,  327 

Hybridisation,  Mendel's  Law  of, 
155;  proof  of,  156;  scope  of, 
159,  160 

Hybrids,  154;  among  Echino- 
derms,  110;  variation  of  struc- 
ture of,  with  season,  111; 
reversion  of,  143;  dominant 
and  recessive  characters  of, 
156;  resolved  into  pure  pa- 
rental forms,  157;  useless  for 
breeding,  161 ;  give  rise  to  new 
characters,  161 ;  relation  of,  to 
parent  forms,  161;  occurrence 
of,  in  nature,  162,  165;  arti- 
ficial, 165;  fertility  of,  165; 
bigeneric,  166;  binordinal,  166; 
among  Lepidoptera,  167; 
among  Echinoids,  168;  with 
zebras,  168;  effect  of  condi- 
tion of  sex-cells  on  characters 
of,  169 


Id,  defined,  103 

Identical     human    twins,     115; 
measurements  upon,  117 


Impregnation,  effect  of  temper- 
ature at  time  of,  191 ;  effect  of 
salinity,  192 

Indefinite  variations,  371 

Index  of  variability,  20 

India,  Lepidoptera  of,  316 

Indices  of  variability,  relations 
between,  28 

Infants,  variability  of,  205; 
curves  of  growth  of,  208 

Infection  of  Germ,  175 

Infusoria,  adaptation  of,  to  high 
temperature,  379 

Inheritance,  direct  and  collateral, 
determination  of,  125;  blended 
and  alternative,  132;  in  man, 
347;  of  Acquired  Characters, 
352;  of  acquired  epilepsy,  364; 
of  adaptation,  in  mould,  377 

Injuries,  non-inheritance  of,  362; 
to  nervous  system  of  guinea- 
pigs,  363 

Internal  secretions,  and  action 
of  environment,  358 

Iron  salts,  effect  of,  on  plant 
growth,  286 

Isolation,  index  of,  42 

Isolation,  physiological,  in  ani- 
mals and  in  plants,  67;  origin 
of,  68 


Japanese  waltzing  mice,  142 
Japanned  peacocks,  172 
Jews,  prepotency  of,  148 


Larvae,  of  Lepidoptera,  effect  of 

food  on,  291 

Larvae  of  sea-urchin,  method  of 
obtaining,    105;    influence    of 
sex-cells  on  size  of,  107;  in- 
fluence of  season  on  size  of, 
109;    reaction  of,  to  environ- 
ment, 190;  metabolism  of,  297 
Latency  of  characters,  156,  159 
Latitude,  effect  of,  on  plants,  311 
Law,  of  Ancestral  Heredity,  122; 
of  Frequency  of  Error,  10;  of 
Hybridisation,  Mendel's,  155; 
of  Reversion,  133 


SUBJECT  INDEX. 


409 


Laws  of  Chance,  dependence  of 
variability  upon,  11 

Laws  of  Variation,  189 

Leaf  surface,  action  of  light  on, 
248 

Lemming,  effect  of  cold  on,  243 

Lepidoptera,  crosses  among,  167; 
sports  among,  171,  172,  175; 
effect  of  temperature  on  size 
of,  230;  reversion  in,  234,  239; 
seasonal  dimorphism  in,  233, 
239,  241,  242;  seasonal  adapta- 
tion in,  234;  seasonal  forms  of , 
produced  by  temperature,  236; 
variable  protective  resemblance 
in,  258;  effect  of  moisture  on, 
268;  effect  of  food  on,  288 

Leuciscus  balteatus,  distribution 
of  fin  rays  in,  42;  variation  in, 
324 

Life,  expectation  of,  347 

Light,  effect  of,  on  plants,  245; 
on  tadpoles,  249;  on  snails, 
249;  on  pigmentation,  249;  on 
Proteus,  251 ;  on  flounder,  251 ; 
on  molluscs,  252;  on  Crustacea, 
253;  on  fish,  254;  on  frog,  255 

Limncea,  effect  of  products  of 
metabolism  on,  302,  303,  307 

Linaria  spuria,  variation  in,  15 

Linaria  vulgaris,  origin  of  muta- 
tion in,  61 

Local  race  of  mice,  350 

Local  races,  the  result  of  environ- 
ment, 310;  in  trees,  311;  in 
Alpine  plants,  311 ;  in  shrimps, 
318;  in  mackerel,  319;  in 
herring,  322;  in  fish,  324;  in 
birds,  325;  in  mammals,  327; 
in  rabbit,  330 

Longevity,  347;  and  fertility,  348 

Lowland  plants,  compared  with 
highland,  311 

M 

Mackerel,  local  races  in,  319 
Maize,  cumulative  action  of  con- 
ditions of  life  on ,  352 
Malformations,  artificial  produc- 
tion of,  173 


Man,  rate  of  growth  of  embry- 
onic, 201;  variability  of,  205; 
permanent  effect  of  environ- 
ment on  growth  of,  208;  effect 
of  light  on  skin  of,  250 

Manures,  effect  of,  on  growth  of 
crops,  282 

Mare,  Lord  Morton's,  175 

Maritime  plants,  269 

Maturity  of  sex-cells,  variation 
in,  110,  112 

Maximum  temperature,  of  plant 
growth,  228 

Mean  Squares,  method  of,  25 

Measurements,  deviations  of, 
accord  with  Law  of  Error,  12 

Mechanical  strains,  adaptation 
to,  375 

Median,  defined,  17 

Mediocrity,  regression  towards, 
126, 129;  of  children,  compared 
with  parents,  129 

Melanism  in  Mollusca,  232 

Mendel's  Law  of  Hybridisation, 
155 

Meristic  variations,  53 

Metabolism,  effects  of  products 
of,  296;  specific,  296,  298,  309; 
of  molluscs,  302,  307;  of  tad- 
poles, 306;  of  DapJmia,  308; 
products  of,  and  action  of 
environment,  358 

Mice,  waltzing,  reversion  in,  142; 
acclimatisation  in,  386 

Mid-parent,  127,  131 

Migration,  effect  of,  on  vari- 
ability, 219 

Mode,  defined,  33 

Modifications,  somatic,  223,  242, 
244 

Moisture,  effect  of,  on  barley,  262; 
on  water-reed,  264;  on  vetch, 
265;  on  Mare's  tail,  266;  on 
water  crowfoot,  267;  on  Lep- 
idoptera, 268;  on  molluscs,  269 

Molluscs,  acclimatisation  of,  to 
saline  solutions,  383 

Molluscs,  effect  on,  of  tempera- 
ture, 231;  of  light,  252;  of 
lime,  287;  of  products  of  me- 
tabolism, 302,  303,  307 


410 


SUBJECT  INDEX. 


Mongrels,  reversion  of,  143 
Monsters,    artificial    production 

of,  173 
Moths,  effect  of  food  plants  on, 

289 
Moulds,  adaptation  in,  375,  377, 

378 

Mouse,  evolution  in,  350 
Mutation,   DeVries'    theory    of, 

59 
Mutations,  produced  by  keeping 

seed,  114 
Mutilations,  non-inheritance  of, 

362 
Mytilus,  effect  of  light  on,  252 


N 


Naquada  race,  correlation  in, 
83 

Natural  Selection,  action  of  on 
variations,  335;  validity  of, 
336;  in  crab,  337;  in  sparrow, 
341;  in  mollusc,  345;  in  man, 
347;  in  mouse,  350;  and  so- 
matic variations,  367;  necessary 
in  spite  of  adaptation,  391; 
produces  spines  in  plants,  391 

Nature,  hybrids  occurring  in, 
162,  165 

Negative  correlation,  73,  79 

Nereis  limbata,  variation  in,  35 

Nervous  system,  effect  of,  on 
skin  colour,  256,  257,  259 

Nigella  Hispanica,  variation  in 
seed  capsules  of,  89 

Nitrates,  effect  of,  on  larvae,  301 

Nutrition  of  plants,  effect  of,  on 
development,  200 

Nutrition,  effect  of,  on  germ- 
plasm,  102, 104, 112;  on  crops, 
282;  on  five-leaved  clover,  284; 
on  Crowfoot,  284;  on  Ficaria, 
285;  on  molluscs,  287;  on  bees, 
287;  on  aphides,  288;  on  Lepi- 
doptera,  288-290;  on  larvae, 
291;  on  tadpoles,  292;  on  birds, 
293,  295;  on  mammals,  294; 
on  spiderwort,  313;  on  Satur- 
nia,  317 


O 

(Enothera  Lamarckiana,  muta- 
tions in,  60,  61;  effect  of  nutri- 
tion on,  200 

Optimum  temperature  of  growth, 
227,  228,  230 

Orchids,  hybrids  among,  166 

Organic  stability,  position  of,  170 

Organisms,  reaction  of  growth 
of,  to  surroundings,  189;  vari- 
able reaction  of,  to  environ- 
ment, 197;  variability  of  grow- 
ing, 204;  variability  of,  in 
relation  to  environment,  218; 
variable  reaction  of,  to  temper- 
ature, 230 

Otter,  variability  of,  221 

Ova,  rabbits',  transplantation  of, 
119;  reaction  of  impregnated, 
to  temperature,  191,  194 

Oxalis,  variation  in,  118 


Palcemonetes,  variation  in  rostral 
teeth  of,  27 

Parallel  variation,  96 

Parasites,  influence  of,  on  crab 
and  on  earwig,  40 

Parents,  contribution  of,  to  char- 
acters of  offspring,  122;  rela- 
tion of  to  offspring,  126 

Parthenogenesis,  in  an  ostracod, 
177;  indaphnia,  178;  in  aphis, 
179;  variability  in,  180;  affected 
by  nutrition,  288 

Peacocks,  japanned,  172 

Pedigree  stock,  in  relation  to 
Law  of  Heredity,  134 

Peloric  flowers,  distribution  of, 
in  Linaria  spuria,  15 

Peloric  race,  origin  of,  in  Linaria 
vulgaris,  62 

Penycuik  experiments,  140,  148, 
168,  176 

Periodic  selection  in  mollusc, 
345 

Periwinkle,  variability  of,  206, 
214 

Phylogenetic  forms,  produced  in 
Lepidoptera,  237 


SUBJECT  INDEX. 


411 


Physiological  characters,  subject 
to  variation,  71 

Physiological  Selection,  66 

Pieris  napi,  effect  of  temperature 
on,  239,  241 

Pigeons,  correlation  in,  85;  re- 
version in,  140;  effect  of  meat 
on,  295;  effect  of  domestication 
on,  333 

Pigment  cells  in  skin,  of  Proteus, 
251;  of  flounder,  251;  of  frog, 
256;  of  Octopus,  257 

Pigments  in  larvae,  derived  from 
food,  291 

Pigmentation,  effect  of  light  on, 
249,  251,  252;  in  man,  inherit- 
ance of,  361 

Pisum  sativum,  varieties  of, 
155;  cross-fertilisation  of,  155, 
159 

Plants,  variations  in,  do  not  agree 
with  Law  of  Error,  15;  de- 
termination of  variations  in, 
46;  hybrids  among,  161; 
diminishing  effect  of  environ- 
ment on  developing,  199; 
desert,  263;  aerial  and  aquatic, 
265;  adaptation  of,  to  changed 
environment,  267 

Plant  structures,  origin  of,  265 

Pleuronectes  flems,  measurements 
of,  14;  variability  in  fin-rays 
of,  27,  34;  correlation  in,  84; 
effect  of  lighten,  251 

Plymouth  Sound,  changing  con- 
ditions in,  338 

Polygon  of  Variation,  17 

Polyommatus  phlceas,  reaction  of, 
to  temperature,  234,  241 

Poppies,  variation  in  seed  cap- 
sules of,  89 

Porto  Santo  rabbit,  330 

Portunus  depurator,  variation 
of,  9 

Position  of  organic  stability,  as 
regards  sports,  170 

Potato,  asexual  propagation  of, 
184 

Prawns,  measurements  of,  14 

Pregnancy,  condition  of  mother 
during,  209 


Prepotency,  in  dog,  148;  in  bull, 
148;  in  horse,  148,  149;  in 
man,  150;  in  Basset  hounds, 
151;  in  relation  to  sex,  151; 
produced  by  inbreeding,  152; 
conforms  to  Law  of  'Heredity, 
154 

Primrose,  variation  in,  48;  hy- 
brids of,  164 

Primula  officinalis,  variation  in, 
48 

Probability  curve,  10 

Probability,  Laws  of,  govern 
variations,  12 

Probable  Error,  17;  determina- 
tion of,  19 

Products  of  metabolism,  effect  of, 
on  growth,  296,  298,  302,  303, 
307,  308 

Protective  resemblance,  255,  260 

Proteus  anguineus,  effect  of  light 
on,  251 

Protophyta,  adaptation  of,  to 
high  temperature,  380 

Pseudoclytia  pentata,  variation 
in,  90 

Pupae,  effect  of  temperature  on, 
240;  effect  of  lighten,  257 

Pupal  stage,  in  relation  to  tem- 
perature, 240;  in  relation  to 
light,  257 

R 

Rabbits,  effect  of  nutrition  of 
sex-cells  in,  113;  transplanta- 
tion of  ova  in,  119;  local  race 
of,  330 

Race,  variability  of,  compared 
with  that  of  species,  182;  local, 
of  mice,  350 

Races,  local,  of  Chrysanthemum 
segetum,  50;  constant  correla- 
tion in,  79,  80;  of  Lepidoptera, 
237;  the  result  of  environment, 
310;  in  trees,  311;  in  Alpine 
plants,  311;  in  shrimps,  318; 
in  mackerel,  319;  in  herring, 
322;  in  fish,  324;  in  birds,  325; 
in  mammals, 327;  in  rabbit,  330 

Ranunculus  repens,  variation  in, 
16,30 


412 


SUBJECT  INDEX. 


Reaction  of  organisms  to  environ- 
ment, variable,  197 

Recessive  characters  in  hybrids, 
156,  159 

Reciprocal  crosses,  112 

Regression,  in  sweet-peas,  126; 
in  man,  127;  coefficient  of, 
127,  128,  132;  towards  medi- 
ocrity, 129;  weeded  out  by 
selection,  154;  in  Daphnia 
magna,  178 

Relative  Probable  Error,  an 
index  of  variability,  20,  21 

Reproductive  Selection,  87;  in 
sparrow,  214 

Reproductive  system,  correlation 
with,  86;  effect  of,  on  internal 
secretions,  94;  effect  of  en- 
vironment on,  221 

Resemblance,  protective  and 
aggressive,  255 

Retardation  of  growth,  persist- 
ence of,  204,  207,  208 

Reversion,  Law  of,  133;  in  man, 
139;  in  dog,  139;  in  calf,  140; 
in  pigeons,  140;  in  fowls,  142; 
in  mice,  142;  produced  by 
crossing,  143;  attempted  ex- 
planation of,  144;  in  Lepidop- 
tera,  234,  239 

Ricin,  acclimatisation  to,  386 

Ripeness  of  sex-cells,  variation 
in,  110,  113 

Rock-pigeon,  reversion  to  charac- 
ters of,  140,  141 

Roots,  interchange  of,  with 
stems,  376 

Rothampstead  experiments,  282 

S 

Saline  solutions,  acclimatisation 
to,  382,  383,  384 

Salinity,  effects  of,  on  develop- 
ment, 192, 199;  on  plants,  269; 
on  garden  cress,  270;  on  frogs, 
271;  on  worms,  271;  on  Arte- 
mia,  271,  274;  on  common 
cockle,  275;  on  Littorina,  278; 
on  Tubularia,  278;  on  sea- 
urchin  larvae,  279 


Sandwich  Islands,  snails  of,  315 

Sciurus  carolinensis,  variation 
of,  4 

Season,  effect  of,  on  size  of 
larvae,  109;  on  structure  of 
hybrids,  111 

Seasonal,  variation  in  size  of 
larvae,  109;  dimorphism  in 
Lepidoptera,  233,  239,  241, 
242;  forms,  among  Lepidop- 
tera, produced  by  temperature, 
236,  241,  242;  adaptation,  234, 
241,  243 

Sea-urchin  larvae,  relation  of 
growth  of,  to  salinity,  279 

Sea-urchins,  specific  metabolism 
of,  296,  298 

Secretions,  internal,  affect  repro- 
ductive system,  94;  and  en- 
vironment, 358 

Seed  capsules,  effect  of  nutrition 
on  size  of,  200 

Selection,  effect  of,  on  variability, 
182;  in  sparrow,  213;  effect  of, 
on  breeding  true  to  type,  153; 
artificial,  effect  of,  on  vari- 
ability, 211,  222;  Genetic,  87; 
Germinal,  395;  Periodic,  in 
mollusc,  345;  Physiological, 
66 

Selection,  Natural,  effect  of,  on 
variability,  211;  action  of,  on 
variations,  335;  validity  of, 
336;  in  crab,  337;  in  sparrow, 
341;  in  mollusc,  345;  in  man, 
347;  in  mouse,  350 

Sex,  effect  of,  on  correlation,  82, 
83;  and  prepotency,  151; 
function  of,  in  evolution,  180, 
185 

Sex-cells,  effect  of  nutrition  on, 
104;  staleness  of,  105 

Sheep,  effect  of  feeding  on,  294 ; 
effect  of  conditions  of  life  on, 
354 

Shells,  variability  of,  206,  214; 
of -cockle,  affected  by  salinity, 
275 

Shore  plants,  269 

Shrimps,  measurements  of  local 
races  of,  13 


SUBJECT  INDEX. 


413 


Sinapis  alba,  effect  of  light  on 
growth  of,  245 

Skew  curves  of  distribution,  29, 
34 

Skull  capacity,  of  rabbit,  332 

Snail,  upon,  relative  growth  of, 
302,  303,  307;  variation  in, 
314,  315;  periodic  selection  in, 
345 

Soma,  effect  of  environment  on, 
223 

Somatic  modifications,  223,  242, 
244 

Somatic  variations,  influence  of, 
on  germ-plasm,  360;  impor- 
tance of,  366, 390;  adaptiveness 
of,  389 

Somatogenic  variations,  101, 223, 
242,  244;  laws  governing,  189 

Sparrow,  variability  of  English 
and  American,  212;  natural 
selection  in,  341 

Species,  and  varieties,  differ- 
entiation of,  41;  a  precise 
criterion  of,  44;  origin  of,  by 
mutation,  59;  produced  by 
hybridisation,  164 

Specific  Gravity  of  waters,  effect 
of,  on  Artemia,  272;  on  Tubu- 
laria,  278 

Specific  metabolism,  in  animals, 
296,  298,  309 

Sphcerechinus  granularis,  dimin- 
ished fertility  between  varie- 
ties of,  71 

Spines,  development  of,  in  plants, 
263;  produced  by  natural 
selection,  391 

Sports,  arising  in  CEnothera 
Lamarckiana,  61;  nature  of, 
170;  stability  of,  170;  among 
Lepidoptera,  171;  prepotency 
of,  171;  among  sheep,  172; 
among  peacocks,  172;  obey 
Law  of  Heredity,  172;  pro- 
duction of,  173;  artificial  pro- 
duction of,  in  Lepidoptera, 
175;  among  Lepidoptera,  237 

Squirrels,  variation  in,  329 

Stability,  organic,  position  of 
170 


Staleness  of  sex-cells,  effect  of, 
on  development,  105,  107 

Standard  Deviation,  defined,  26 

Statistical  methods,  36 

Stature,  of  parents  and  children, 
130;  of  school  Children,  202; 
human,  variability  of,  205 

Stems,  interchange  of,  with  roots, 
376 

Sterility,  between  varieties,  69; 
between  races  of  men,  70;  of 
atypical  forms,  89,  90;  pro- 
duced by  changed  conditions 
of  life,  94 

Sterna  hirundo,  variation  of,  7 

Sternum,  diminished  by  disuse, 
333 

Strongylocentrotus  lividus  larvae, 
measurements  of,  14;  figures 
of,  105;  reaction  of,  to  temper- 
ature, 193,  194;  effect  of  en- 
vironment, on  variability  of, 
217 

Substantive  variations,  53 

Subterranean  caves,  animals  in, 
254 

Sunlight,  effect  of,  on  growth  of 
plants,  246 

Surroundings,  effect  of,  on  vari- 
ability, 218,  220;  effect  of,  on 
growth,  189;  adaptation  to 
changed,  390 

Swine,  variation  in  Mullerian 
glands  of,  25 


Tadpoles,  development  of,  in 
relation  to  temperature,  225; 
effects  of  feeding  on,  292; 
acclimatisation  of,  to  tempera- 
ture, 385 

Telegony,  in  mare,  175;  negative 
results  regarding,  176;  statisti- 
cal examination  of,  176 

Temperature,  effect  of,  on  growth 
of  Echinoderm  larvae,  190, 
194,  196;  variable  effect  of, 
on  growth,  196;  diminishing 
effect  of,  with  development, 
197;  fatal,  on  developing 


414 


SUBJECT  INDEX. 


larvae,  198;  effect  of,  in  pro- 
ducing variations,  224;  effect 
of,  on  development  of  frog, 
225,  226,  227;  optimum,  227; 
maximum,  228;  effect  of,  on 
plant  growth,  228;  on  size  of 
Echinoid  larvae,  229;  on  Lepi- 
doptera,  230,  233,  239,  242;  on 
Mollusca,  231;  on  lemming, 
243;  on  hare,  244;  adaptation 
to  high,  in  Flagellata,  379 

Temperatures,  death,  of  develop- 
ing larvae,  198 

Torilis  anthriscus,  variation  in, 
15,46 

Toxin,  diphtheria,  acclimatisa- 
tion to,  387 

Transplantation  of  rabbits'  ova, 
119 

Trotting  horses,  prepotency  in, 
149 

Twins,  identical,  115;  measure- 
ments upon,  117 

Type  forms,  fertility  of,  90; 
replacement  of,  by  variety,  91 

Typha,  measurements  upon,  42 


Umbelliferae,  variation  in,  48 
Undifferentiated  like  organs,  135 
Unpigmented  cave  animals,  254 
Urea,  effect  of,  on  larvae,  300 
Uric  acid,  effect  of,  on  larvae,  300 
Urmi,  Lake,  Artemia  present  in, 

274 
Use  and  disuse,  inheritance  of 

effects  of,  360 


Vanessa  cardui,  production  of 
sports  of,  175;  effect  of  tem- 
perature on,  237 

Vanessa  io,  production  of  sports 
of,  175;  effect  of  temperature 
on,  237,  238 

Vanessa  levana,  seasonal  di- 
morphism in,  233 

Vanessa  prorsa,  seasonal  di- 
morphism in,  233 


Vanessa  urtic®,  effect  of  temper- 
ature on,  237;  protective  re- 
semblance in,  258 

Variable  protective  resemblance, 
255,  260 

Variability,  diagrammatically 
represented,  5;  method  of 
estimation  of,  17;  Law  of 
Equable,  96;  in  parthenogene- 
tic  forms,  177,  180;  in  poppies, 
181, 182;  individual,  in  relation 
to  racial,  181;  racial  and  spe- 
cific, 182;  small,  of  a  sexually 
reproduced  forms,  184,  185; 
relations  of,  to  external  condi- 
tions, 199;  of  growing  guinea- 
pigs,  204;  diminution  of,  with 
growth,  204;  of  growing  man, 
205;  of  crabs,  206;  of  peri- 
winkle, 206,  215;  of  embryos, 
207;  diminution  of ,  with  evolu- 
tion, 210;  diminution  of,  with 
selection,  210;  of  offspring  and 
parents,  210;  relation  of,  to 
homotyposis,  211;  relation  of, 
to  intensity  of  heredity,  211; 
effect  on,  of  artificial  selection, 
211;  effect  on,  of  Natural  Se- 
lection, 211;  effect  of  fertility 
of  type  forms  on,  212;  of  Eng- 
lish and  American  sparrow, 
212;  of  environment  and  of  or- 
ganisms subjected  to  it,  219;  in 
relation  to  Domestication,  221; 
of  otter,  221 ;  produced  by  food , 
281;  produced  by  season,  285; 
of  sparrow,  342;  increased  by 
somatic  variations,  368 

Variants,  defined,  4 

Variation,  defined,  1;  numerical 
treatment  of,  3;  universally 
present,  6;  obeys  Laws  of 
Chance,  11;  importance  of 
numerical  treatment  of,  98; 
hereditary  cause  of,  102;  due 
to  amphimixis,  103,  114,  126; 
of  organisms,  chiefly  blasto- 
genic,  121 ;  produced  by  cross- 
ing, 161;  Laws  of,  189;  in 
periwinkle,  215,  in  birds, 
219 


SUBJECT  INDEX. 


415 


Variations,  swamped  by  inter- 
crossing, 57,  58;  relation  of, 
to  variable  environment,  197, 
199;  and  modifications,  223; 
adaptive,  371;  definite  and 
indefinite,  371 

Varieties,  sterility  between,  69 
Variety,  replacing  type  form,  91 
Ventricosity,   variability    of,  in 

periwinkle,  206,  215 
Vetch,  effect  of  water  on,  265 
Volume  of  water,  effect  of,  on 
growth  of  snail,  302;  of  tad- 
pole, 306;  of  Daphnia,  308 


W 

Waltzing  mice,  reversion  in,  142 
Warmth,  effect  of,  on  Lepidop- 

tera,  233 
Weight,  human,  variability  of, 

205 
Willow,  interchange  of  root  and 

stem  in,  376 
Wolf,  variation  in,  328 
Woman,  variability  of,  205 


Zebras,  crosses  with,  168 


THB   END. 


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